JL911

I have a 1999 996 6spd and am having a weird issue where when I am slowing down a scraping/creaking noise is emanating from the rear of the car. It will continue making the noise, its frequency slowing as the car slows, regardless if I am pressing the brake pedal or not. Finally it stops when the car comes to a stop. It seems independent from the engine as putting the trans in neutral or revving the engine in neutral as I am slowing down does not change the noise. Also pulling the ebrake doesn't seem to make a difference either. I am not sure if this is an axle or wheel bearing since when I am accelerating or going at a constant speed the noise goes away. It seems to only make the noise when slowing down.

cds72911

Bent brake backing plates and/or stones trapped against the rotor have had these symptoms before.

JL911

But would this explain the noise going away while accelerating?

sasilverbullet

Yes, because the stone could angle differently.

Other possibilities: rear brakes, cv joints, wheel bearings.

Imo000

jack up each corner one at a time and spin the tire......see if you can hear the noise. You'll have to get your hand dirty to properly look at this. If not a stone between the rotor and the dust shield then maybe the parking brake hardware is coming apart.

dlknight

Another one I haven't seen mentioned is A/C compressor, the make a horrible scraping mechanical sound on overrun if there is a leak in the system as the compressor will not be getting lubricated. Simple test is to turn off the A/C and if the sound goes away you've found your culprit.

Imo000

If the AC is leaking the pressure switch will not allow the compressor to turn on.

Scott at Team Harco

Would also not be vehicle speed-dependent.

NuttyProfessor

Same thing happened to me on the way to walmart. Heard a weird noise coming from back of our SUV. Got to Walmart and my little daughter said, "Daddy, I hear a cat!" I looked under the spare tire and there was our cat Tippy. Darn thing rode all the way there without falling out. hahahaha

808Bill

Dude, didn't you recently mention your dog in another thread? Lol!

NuttyProfessor

Probably, but I don't even remember what I ate for breakfast much less any of my dog posts. btw, my dog is virtually a mouse in a dog's body. My wife had to have her, but she's family now. But sad watching a dog get beaten up all the time. It's scared of it's own shadow. I hate to see how it would perform in a real dog fight.

Imo000

dlknight

Really, well I definitely have an A/C leak from the usual area in front of the rear wheel arch on the right hand side and my A/C still turns on. Plus the specialist that did my pre-purchase inspection pointed out that the A/C compressor was noisy due to a leak. And like I said the noise goes away when I turn off the A/C.

Well it kind of is and could be easy for someone to associate the sound with road speed when it could be engine speed dependent, with my compressor noise at least it doesn't do it when I'm accelerating only when I come off the throttle and slow down.

It is an easy test as I said above - turn off the A/C and drive a little - if the noise has gone then you have your culprit - otherwise good luck finding the issue.

Welcome to the internet where everyone else knows better!

Imo000

I don't think this specialist knows what he/she is talking about. If your AC still works then it isn't leaking. If there is a leak, even a minor one, doesn't take much for the gas to escape and the pressure switch to disable the system. You might get a season out of it if the system is full and the leak is very small but sooner or later it will drop below the threshold of the pressure switch and at the point the compressor will not engage anymore. Your compressor noise is separate from a leak.

Why don't you educate yourself so you don't have to rely on what "experts" say?

dlknight

I don't need to educate myself thank you very much - you can look at my 944 Turbo Rebuild thread for my credentials, what are your credentials? The specialist is a well known Porsche independent garage in the UK with ex-Porsche technicians.

Perhaps the compressor noise is separate from the leak but my original suggestion of turning off the A/C is such a simple test why do you need to argue?

A photo of my A/C leak -