knocking noise over 50km/h

[marcdon]

hi guys.
VZ 3.6L 4spd Exec
Recently I have noticed a knocking noise (more of a thud...thud...thud...thud) coming from under the car when my foot is off the accelerator and speed is around 60km/h and over. The noise is pretty quiet but you can hear it when the radio is off and windows up. the faster you go the quicker the "thud" gets. My gut keeps telling me to look at the driveshaft or diff?
what could it be and/or has anyone else had this problem?
Diff oil gets changed every 20000kms
Transmission every 50000km

[showxpony]

That's a good question for your local mechanic. But I hear a similar noise in the VZ calais cruising at 60 in 4th (5speed box). If i keep the car in 4th and cruise at 60 or over and let go of the accelerator theres a very faint winding/thuding type of noise, but change it up to 5th and i hear nothing at all. I just had a major service done, and everything was changed.. seems to be smoother now, but i think just being in that lower gear has a different effect on the back end. But you would have the 4 speed box in the exec.. so longer gears shouldnt hear it at 60 (you'd think). None the less, i highly doubt its something to worry about. From experience, if the sound isn't distinct and loud, theres normally no problem.

[marcdon]

ok thanks mate

[showxpony]

Just thinking about it, it may be the noise of the diff gears slowing down, as the sound only comes up when travelling over 60kph and getting the gears to spin at a certain speed, then being slowed down by the engine with the foot off the accelerator, and the sound is the gears being slowed down a bit quicker than normal due to the engine slowing the back end down... does that make sense? haha. Try cruising at 60, put the car into neutral and see if the sounds still there. I'll have a go as well and report back.

[calay]

could be cv joint, wheel bearing etc but get it checked out cos its only gonna cause more damage the longer you leave it

[showxpony]

Hey buddy.. had a go at playing with the gears and decelerating at 60, and i couldnt hear a thing! but then i realised something... my left CV shaft broke last year, and i got it changed.. now, i can't hear any knocking.. maybe its your cv shaft/joint like -Calay has mentioned. When you accelerate over 2500/3000rpm, can you hear a clicking or thudding from the rear end? Could be a CV problem... If you want to check more physically, jack the car up and twist the CV shafts by hand, they should be rock solid, if the have a bit of give and can rotate back and forth, then the shafts rooted.

[bigdaddycool]

I'll put this out there cause I have no idea what it sounds like......... but check the exhaust shields at the rear........ I must of had something flick up under the car on mine at one stage and started replacing gear thinking it was my wheel bearings, cv joint........ brake rotars, brakes........ turned out to be the heat shield banging ........ i checked under the car twice over and missed it, 3rd time........ i found it as i couldn't see it looking from a certain angle with the driveshaft in the way.....

......

[rb0135]

Its called Cyclic Knocking and there is a techline to fix it.

If you have warranty, you're in luck as it is a known issue and all Dealerships know about it.

I fought for 3 years to get my dealer to accept the issue. Then they miraculously found a techline article on it.. Lucky I had 6 year warranty.

It has to do with too much play in the splines of the axles that fit into the diff. and the bolts that hold the axle on need to be loctited properly.

I dont know how much it would cost to fix out of warranty. I did post an article on this forum a while ago, but it doesnt seem to be around now.

Rob