BobbyT

I’ve noticed a rhythmic sound from my 993’s engine, a low-pitched sort of “rrrRRRrrr”, at about one Hertz frequency, but only when the engine is at constant speed of about 3,000 RPM, level road, steady throttle, in any gear 2 to 6. It isn’t very loud, just noticeable, with no change in engine feel or RPM.

I thought this might somehow be related to the O2 sensor, having read some comments about this sensor getting “slow”. Replaced the sensor, no change. Engine has 67,000 miles, full 60,000 service has been done.

Any ideas about what might cause this?

I would make an audio recording, but I don’t think it is loud enough to register…

pp000830

Here is one idea...
The engine vibrates one frequency, the transmission rotating gears at another the vibrations constructively and the destructively interfere as they move in and out of phase producing a very low frequency pulsing like sound.

Other's thoughts?

Tlaloc75

I've heard a similar noise. I have RS mounts and chalked it up to frequency interference as you suggest PP. It only happens at certain RPMs and only on certain days, so perhaps temp related. It may even be related to tire/road frequency interacting with the engine note.

I find it annoying if I pay attention, but otherwise it fades out of my mind and next time I drive its gone! Just another quirk of the 993 and a reminder of my OCD tendencies .

nine9six

3,000 RPM, any gear?

BlackSnake99

It's your car whispering 'go faster...'.

BobbyT

nine9six,

I haven't observed it in 1st or reverse, but it's there in 2nd through 6th, only in a very narrow RPM range. It begins after 2800 RPM and by 3200, it's gone.

pp's reply suggests "harmonic dissonance". Sounds like the cure may be to turn the music up...

Thanks for the replies. Trying to be sure something bad isn't happening.

Ivan J

I experienced a similar symptom when my fan hub bearing started going bad.

nine9six

I wanted to make sure it wasn't a tire resonance... how long has the sound been present?

BobbyT

nine9six,

I've been noticing it for a couple of months.

Ivan J,

I'll check the fan to see if I can detect any slop or signs of rubbing. Anything else to check there?

Ivan J

Mine looked fine, statically. There was no evidence that the fan had scraped on the shroud. I continued to drive it and wonder about the source of the sound for several months. Eventually, I removed the belts and found that when I spun the fan by hand I could see a little bit of wobble. I converted to the RS single-pulley setup which eliminates the fan hub bearing, and the low frequency noise went away.

IainM

that's what I was thinking too
any chance you activated ABS since you got these tires? I wonder if the flat spot pattern caused by ABS could contribute to low freq noise?
otherwise how is a difference frequency being generated in a straight line at constant speed- can't see how different rotational parts can beat like that

BobbyT

Ivan J,

My next step will be to remove the belts and spin the fan, check for wobble or looseness, any bearing issues.

IainM,

I haven't invoked the ABS since the current tires were mounted, and the noise shows up in all gears 2 through 6, at the same RPM, so maybe not related to tires.

Thanks guys, I appreciate all the help.

BobbyT

Removed fan and alternator belts. Alternator spins smooth and quiet, no slack motion in any direction. Cooling fan can be wobbled slightly, about 1 mm, and there are some marks indicating fan contact on the lower part of the fan shroud. I think the fan hub bearing is failing, so a new one is en route.

993 thanks, guys. This a great place!

nine9six

Kudo's Ivan, for hitting the nail on the head!

OverBoosted28

I find that discerning slight changes in any engine noise can be maddening. It takes a while, after hearing something, to realize it's nothing, I'm going to be able to readily define. Just this weekend, while driving to track, I'm like "why does the rear end sound loud"? Then realized it's just the dumb R888s that make so much noise. Unless there's other symptoms than "noise", it's hard to tell what it could be. Just drive!!!!! Deal with issues as they arise, don't do as I, and expect the worst, at the worst possible time! :-0