Hum behind instrument panel
02-03-2004, 03:08 AM
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Hum behind instrument panel
Hi!
I'm concerned about a noise that recently started making its apHearance. It almost sounds like a power steering without any fluid, but is located (or so I think) to the right of the instrument panel (here in South Africa we have right-hand drive). I noticed that it normally comes on between 60 and 70 km/h and not every time.
Could this be a normal vibration or maybe something more serious like the alternator?
I'm not mechanically inclined, so I don't know how to go about fault finding...
I'm concerned about a noise that recently started making its apHearance. It almost sounds like a power steering without any fluid, but is located (or so I think) to the right of the instrument panel (here in South Africa we have right-hand drive). I noticed that it normally comes on between 60 and 70 km/h and not every time.
Could this be a normal vibration or maybe something more serious like the alternator?
I'm not mechanically inclined, so I don't know how to go about fault finding...
02-04-2004, 08:50 AM
#5
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Edwill,
-for a shot in the dark ( if more precision is req'd, an improved descriptive would help), I had a similar prob. on a 5-series BMW: at ~125KPH, a 'buzz' would start and quickly progress to a deep, intense moan - located deep within the right dash.
For a period too long to admit, could not find the cause in any mechanical system - for it sounded like a bearing 'moan'. What it turned out to be was a vibrating piece of cowel trim (air intake for the blower) just under the hood near the firewall: Some new plastic BMW fasteners cured the noise).
In a RHD 928, the rather flimsey plastic trim over the wiper motor is unsecured on the windshield side and may be buzzing about. Take a look under the hood - you never know....
-for a shot in the dark ( if more precision is req'd, an improved descriptive would help), I had a similar prob. on a 5-series BMW: at ~125KPH, a 'buzz' would start and quickly progress to a deep, intense moan - located deep within the right dash.
For a period too long to admit, could not find the cause in any mechanical system - for it sounded like a bearing 'moan'. What it turned out to be was a vibrating piece of cowel trim (air intake for the blower) just under the hood near the firewall: Some new plastic BMW fasteners cured the noise).
In a RHD 928, the rather flimsey plastic trim over the wiper motor is unsecured on the windshield side and may be buzzing about. Take a look under the hood - you never know....
02-04-2004, 09:21 AM
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Thanks Garth, I'm glad to hear there's a possibility of it being something small - something I'm not used to, normally it ends in $$$$
I'll check...
I'll check...
02-04-2004, 08:49 PM
#7
Honest! 
No sarcasm intended.
I'm with Garth on this one. FWIW/IMO
I don't believe there is anything close enough to the dash area you mentioned that could make a sound that would appear to be coming from inside the sHARk. I drive heavy equipment and over the years noticed that the deep hum and 'Bearing squeal' is pretty much always shaft or bearing related -fan/motor/impeller- with the deep hum eventually resonating thru the duct area and finding something to vibrate enough to make you wonder what's about to blow-up or fall off. You really notice it if the fan only operates on the higher settings.
Again, you have made mention of the noise happening, intermittently. at the 60-70kmh range which would give a bit of weight to Garth's suggestion that there could be a loose bit/flap of plastic involved. Might want to check your wheel well splash shields while you are investigating. To support my theory: the fan is one of those forgotten items, reliable, seldom breaks,
and usually just quits rather than dragging out a lingering death.
However, that puppy spins a lot faster than you would imagine to push a fairly large volume of air (please, no new electric supercharger arguements)
for its size thru some relatively large diameter tubes and collector boxes.
When a bearing - motor or shaft - starts going you can end up with a high-speed ocillation.
Since few automotive electric motors are mounted on isolators -why bother the motors a pretty darn smooth- when it does develope an ocillation it is invariably:
1. Mounted to a stout plastic box directly, or by a metal bracket.
2.The plastic box is usually large-ish and also mounted directly -without isolation-.
3. There is usually one or more duct doors also made of plastic in close proximity. Usually controlled by vent lever cables-those sliders on your HAVC control head.
4. These duct doors are usually pivitoted -open and closed- on metal hinge pins. the doors may pivot vertically or horizonally. The doors usually seal with foam trim.
5. All of the un-isolated plastic and sheathed control cables while mounted securely, and obviously tested and enginnered to be quiet when everything is functioning properly turns into a great transmitter of sound and vibration when things don't work as designed.
Hence, deep hums.
If you are in an extremely dusty environment the fan impeller/turbine collects and packs dust like crazy on the blades and that alone can cause a out of balance fan/shaft problem while spinning at speed on the driveshaft which puts us back into the resonance induced hum. Moisture and fungus can cause a fan /shaft imbalance as well.
Good luck on the hunt, great cleaning opprotunity.
No sarcasm intended.
I'm with Garth on this one. FWIW/IMO
I don't believe there is anything close enough to the dash area you mentioned that could make a sound that would appear to be coming from inside the sHARk. I drive heavy equipment and over the years noticed that the deep hum and 'Bearing squeal' is pretty much always shaft or bearing related -fan/motor/impeller- with the deep hum eventually resonating thru the duct area and finding something to vibrate enough to make you wonder what's about to blow-up or fall off. You really notice it if the fan only operates on the higher settings.
Again, you have made mention of the noise happening, intermittently. at the 60-70kmh range which would give a bit of weight to Garth's suggestion that there could be a loose bit/flap of plastic involved. Might want to check your wheel well splash shields while you are investigating. To support my theory: the fan is one of those forgotten items, reliable, seldom breaks,
and usually just quits rather than dragging out a lingering death.
However, that puppy spins a lot faster than you would imagine to push a fairly large volume of air (please, no new electric supercharger arguements)
for its size thru some relatively large diameter tubes and collector boxes.
When a bearing - motor or shaft - starts going you can end up with a high-speed ocillation.
Since few automotive electric motors are mounted on isolators -why bother the motors a pretty darn smooth- when it does develope an ocillation it is invariably:
1. Mounted to a stout plastic box directly, or by a metal bracket.
2.The plastic box is usually large-ish and also mounted directly -without isolation-.
3. There is usually one or more duct doors also made of plastic in close proximity. Usually controlled by vent lever cables-those sliders on your HAVC control head.
4. These duct doors are usually pivitoted -open and closed- on metal hinge pins. the doors may pivot vertically or horizonally. The doors usually seal with foam trim.
5. All of the un-isolated plastic and sheathed control cables while mounted securely, and obviously tested and enginnered to be quiet when everything is functioning properly turns into a great transmitter of sound and vibration when things don't work as designed.
Hence, deep hums.
If you are in an extremely dusty environment the fan impeller/turbine collects and packs dust like crazy on the blades and that alone can cause a out of balance fan/shaft problem while spinning at speed on the driveshaft which puts us back into the resonance induced hum. Moisture and fungus can cause a fan /shaft imbalance as well.
Good luck on the hunt, great cleaning opprotunity.
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02-05-2004, 03:18 AM
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Sounds like I'll be going on a real fault finding expedition, so I'll get to know my car better as well - maybe I'll bring our relationship closer 
02-05-2004, 10:01 AM
#10
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A worn checkvalve can sound like a hum. I saw Wally replace one at the first or second SITW in GA. So, consider the black/blue check valve off the brake booster.