alsaab

Could'nt find any related thread, so here's the question :

does anybody else have this annoying resonance at +/- 2000 rpm. You can only hear it from inside the car, you don't from the outside. It's not car speed related, only engine speed. The sound goes "wham wham wham wham...." just like a bad tire or perhaps bearing.
Btw.: mufflers are stock !!

Hope you can understand my english and greetings from Luxembourg

Alain

Rassel

Do you drive the car frequently, or has it been standing still for a long time?
Do you hear the sound after the engine is warm?

alsaab

Hi Rassel,

I really don't drive the car frequently, but if I do, I drive it for more than one day. The resonance is always there : from cold start to operating temperature during the whole running time.

Hope we'll find the answer !

Thanks

Alain

1995 C4 Ragusagreen/black

willisit

With Aircon or without? Or both?

alsaab

With and w/o AC, no change !!!

Thanks

Alain

Rick Lee

Simple - don't lug your engine. 2k rpms is torture. I shift at 3500-4000 until warm and never under 5000 once warm. My car has never once been lugged since I've had it and I've never used 6th gear.

SundayDriver

There is a natural resonance in the 993 interior that matches ~2300 rpm (if I recall correctly). You can find this with certain mufflers and if you move you head, it can be louder or quieter. This is because you get a standing wave inside the car and you can put your ear at a peak or valley location.

That said, anything the is making a noise around that same frequency can be very annoying in a 993. Even though you have stock exhaust, something that is loose could create the standing wave. Look at muffler mounting, engine/trans mounts, pipes that are loose, etc. Does not have to be something mechanical inside the engine - any source could casue that noise.

jhb

Similar problems have been listed before, both by myself last year and another thread just recently. My problem was combination alternator and bearings that the alternator spins through for the engine fan. Similar problem resolution for the other thread.

krburrell

I also have an annoying resonance but it's at +/- 4200 rpm and goes away with the AC on. Any ideas on what this could be? I searched the archives for "alternator" and "resonance" but did not see anything similar.

alsaab

I just found the "Harmonic hum" thread from jhb and guess what : YES, I DID change my belts and even updated to the new alternator pulley. I just didn't make the link. On the other hand, the belts are correctly tightened by the manual, so............just a wait a couple of miles and see if it settles ??!

Anyway, thanks a lot, at least I know now where to look !!!

Alain

willisit

I had a "hum" at 3000rpm. Turned out to be a loose bolt on the air conditioning compressor...

alsaab

Just finished installing my new (original Bosch rebuild for 262 €) alternator. Two days ago I took the new belts off again and checked the alternator bearings : the fan had already 1 mm play. I also changed the fan bearing (99 € !!!) cleaned the shroud and the fan and now everything's perfect.
But the hum, guess what : the hum is still there at 2000 rpm !!!????
Don't know what to check next!

Alain

Arrwin

Interesting. What happens during traffic? I try my best not to lug the engine as well, but that often times tend to be difficult in NY.

garychios

Its a natural phenomenon. If you want to fix it, put some dynamat on the ceiling, doors and floors of your car. The resonance will go away when changing the natural frequency of the materials. Try another set of stock mufflers as well. As long as your motor is not vibrating, then it just happens to be that the natural frequency of your car is matched with the frequency of your motor at 2000 RPM and bam, double the amplitude and then you hear and feel it.

Yes, I am an engineer with multiple degrees in Mechanics, Aerodynamics, Vehicle design etc. I love thermo the most!

Pete Lech

I have the RSR mufflers and get a mild resonance from 2300-2800 rpm or so. It seems to be increasing in range, which naturally corresponds with typical freeway speeds.

As far as lugging, I don't believe that 2000 rpm at light load is lugging these motors at all. I do think that 3500 rpm on a cold motor is excessive. I try to keep it under 3000 rpm until it warms up.