well ive been driving my datsun a lot lately, i took it on a few 100 mile trips, and well every time i get off the freeway and come to a red light i hear this knocking noise it sounds like coming from somewhere underneath the car. when i rev the engine the knocking goes faster and at higher revs it seems to go away. it only does this when im sitting , not moving, just idling at a red light, and after ive been driving at high speeds on the freeway. is this my exhaust? it seems like its coming from the cat. but im not sure. is it normal? do i need to get new exhaust equipment asap? i just want to make sure im not doing any major damage to my car. thanks guys

if you have a 240 you wouldnt have a cat converter so that would be out of the question. if its loose and or beating against the trans tunnel or rear suspension this is possible.

it could be a number of things.

raise that car up and take a look underneath and see whats going on

I had a similar problem with my first 240z back in '85. It sounded like a ball bearing bouncing around in the pipe right under the floor. It would come and go. Strangest thing is, I put the car up on ramps and rapped on the resonator with a hammer :stupid: (being the mechanical genius that I am!) and it went away. Go figure.

I haven't had the problem with my present 240z...and I am about to replace it completely with a custom job....so probably will never have the problem again!!

FWIW

Zedguy

I've seen at idle the tail pipe taps against the rear panel where it exits the car.Try holding the tailpipe (of a glove is required)and see if it stops.The lift idea is O.K. but I would prefer either a pit or a lift like the muffler shops use so the weight is not taken off the suspension.That could hide the problem on a standard lift.

yah sorry its a 280z so i do have a cat. whats the resonator??

Resonator is like a small muffler usually near the front of the system. Takes the 'vibration' or 'resonence' noise out. I think mine got plugged up with rust or something.

I've had similar problems with the cat. convertor in other cars. Had a VW GTI that plugged up. Made all kinds of noise and then finally I noticed a power loss. Took it to a garage and they took it off and blew it out. Said it was like a birds nest in it. Ran like a charm after....and much quieter!!

Zedguy