Spent many hours searching the 964 threads for anything that comes close to describing my symptoms without any luck, so I reluctantly decided to post a new thread.

Started hearing a loud momentary 'roar' or 'growl' during acceleration through about 3100 rpm. It's very repeatable and lasts about 1 second when accelerating normally. If I maintain a fixed rpm at about 3100, the noise stays as long as I'm around that engine speed. It's not a bad sounding noise, it's just very worrisome because it wasn't there before

Tried isolating it but the best I can do is say it's coming from the engine somewhere (not very useful, I know). I don't suspect any kind of exhaust leak, but the noise resembles what I imagine I'd hear if a hole opened up momentarily in my exhaust system, upstream of the muffler. Seems to happen under load only since I don't hear the noise when revving the engine while in neutral. Performance is unaffected as far as I can tell. Also, gets louder when the engine warms up.

I plan on checking as many vacuum hoses as I can (maybe a vacuum leak is preventing something from opening/closing at that rpm?)

Any thoughts or sharing of previous experiences would be much appreciated.

Oh, the car: 1990 964C2 Manual

one of the heat shields under the exhaust resonating because it got loose a little bit? Common on 964's. Otherwise something else got a little but loose. Your descritption points me in the direction of resonating. Let someone else sit in the car and let it make 3,1k rpm (in neutral with parking brake on) to listen where it comes from (you laying behind it). Maybe there is a little crack in your headers or secondary muffler (if still in place).

Leaky head gasket.

Mine had developed this exact symptom, not an awful noise but definately different to what was there before and almost exactly at 3100 rpm.

A recent trip in for a service has revealed that the final silencer has a small split on the inner side nearest the engine. According to the man with the spanners this is almost certainly the cause of the resonating sound which I have been experiencing.

I priced a new secondary silencer/muffler but due to cost I may also do a tear down and see if I can have it welded for now.

Is it when the A/C is on?

Not in my case.

A/C off.

I thought of something else: Seems louder when warmed up.

Thank you Ghengis! I had suspected an exhaust leak at first since that's what it really sounds like, except that it's momentary, but couldn't find any obvious leaks when crawling underneath at idle rpm. Also, I was convinced that under load, any exhaust leak would get loud and stay loud regardless of rpm's.

I will definitely have a much closer look at the secondary muffler and post if I find anything. I'm hoping that I find something....a new muffler would be better than some other alternatives I've been reading about in various threads.

UPDATE: Lowered my secondary muffler and found a 6 inch vertical crack about midway down the inboard (engine) side. That's got to be it. Wasn't easily visible without lowering, unless you've seen one before and know what you're looking for. Now to fix, or replace. Thanks for the lead Ghengis

Time to go for a G pipe?

Glad to hear that it looks to be nothing more serious. Now of course you face the same dilemma as myself, buy a new muffler or try to weld it up.

I would like to try welding it but it really is just putting off the enevitable. To be honest I've been driving it like this for a few weeks now and I gotten quite used to it. It's only when I've been out for a run and it's ticking over on the drive that I really notice it as it's got a bit more raspy.

Would be nice but I already have the primary bypass. Adding a G/J-pipe as well is going to be way too loud based on other threads I've read.

do you still have the primary muffler? put it back on, and sell/swap the primary for a G-Pipe, problem solved

Don't have the primary muffler. Got the car with the primary bypass installed. The easy fix is repar or replace the secondary

Replaced the muffler yesterday (got a great deal on a new one). Noise is GONE

Still planning on getting the old one welded to have as a spare though.