Rpm related drone/resonance
 

Hi everyone, i'm still fixing my 97 turbodiesel and it's getting better but i'm having a hard time finding this noise. I'm like a droning noise or a resonance that seems to come from the front of the car, around the engine. It's quite intrusive around 1500 and 2000 rpm, loud around 2000 rpm, like a deep intake noise.

The noise has always been there to some degree but i think it is more pronounced since i did the following things last month:

- Rear muffler replacement
- EGR blanking plate
- Fan clutch refill.

Things i've already checked:

- No exhaust leak or loose clamps.
- No tension in the exhaust to transmission bracket.
- Motor mounts and trans mount in good shape, few months old.
- No metal to metal contact between engine, transmission, exhaust and heat shields.
- EGR plate tight and no leaks.
- No play in belt tensioner, pulley and damper.
- Working one way alternator pulley.
- Properly attached air filter housing.

What do you think? Could it be the fan clutch grabbing more than before and creating tension on the belt, simply air gushing through the radiator and condenser, or the EGR used to dampen intake pulses and it doesn't anymore? It's a bit irritating but i'm living with it, i know that this kind of noise is extremely hard to find.

Would the new exhaust hangers bring the exhaust too close to the chassis and create this vibration/resonance?

Filling the fan clutch does increase the noise but that noise should be more of a whooshing sound. Remove the belt temporarily to see if that's the issue.

After driving the car a lot lately, it really sounds like it has something to do with the exhaust, as i can really feel it in the floor and through the pedals.

Yes, changing the muffler can increase drone. Two ways to solve or reduce, install a quieter center resonator or install a J pipe into exhaust system. Google J pipe to learn
about.

I am tempted to remove all heat shields and see if there's an improvement. I can't hear or see anything wrong whith the exhaust outside the car while i have someone holding the throttle to replicate the issue. I can't see or hear any rattle at all in anything related to the v-belt either.