Car is 996tt. Sometimes if I want to accelerate while the car is cruising at a low speed (around 20 to 30 mph), I'll hear a knocking sound at the back of the car with a little hesitation. My mechanic said it may be a backfire. What do you think? Have you guys experienced this before and if so how to fix it?

 

I hope your using 93+ octane...

 

I have been using 98+ octane gas. It doesn't happen all the time. But when it happens, the engine area will knock and car will hesitate before it accelerates.

 

Actually, it is more like a "thump" at the back (with a slight hesitation) when accelerate from a cruising speed?

 

see mechanic!

 

The prob is that the mechanic has no clue as to what has been causing the rear end clunk sound when accelerate quickly during low speed cruising. He said everything (mounts, suspension and sway bar) is tight.

I've seen some web saying that the noise could be from a faulty transfer box or torque converter, any views?

 

What about the elctronics? Did the mechanic check that stuff too?

Are autos managed with computer?

 

I think someone said that this was caused by a loose rear sub-frame. I too have encountered this problem from time-to-time.

- KJ

 

I've already asked the mechanic to check the subframe and he said they are rock solid.

 

Ooops, previously I meant 91+ octane not 93 :-/

 

Sorry guys, I really hope that I can get some expert views from the enthusiasts from this site.

To give a better description of the problem. The sound is more like a metallic "thunk" from the rear around the engine area (it seems to come from underneath the engine, maybe from the exhaust). It doesn't happen all the time. Sometimes it happens when the car accelerate (not WOT) at a cruising speed of around 30 to 40 mph (at 3rd gear and sometimes 4th gear, probably around 2000 rpm). The thunk sound does not seem to be from the transmission coz it is from the rear end of the car. Mech thinks that it could be a backfire. I am not sure if this "thunk" sound is actually describe by other people as "ping" in the engine (spark shock). Since I have been using high level octane gas so the sound shouldn't be the sound of a detonation (plus it doesn't happen all the time). Any comments or views?

 

Mech said to me today that the car has a slight backfire when accelerating from a low speed. He has not yet been able to locate the cause of the backfire. Any suggestion?

 

Tie the mechanic to the back of the car while u accelerate. He'll locate it.

 

Ha! Ha! ....

 

A backfire may show up in misfire recognition, detonation would show up in knock registration. axles tight?