Rumbling as car slows down at 1500 rpm-is - not brakes

[step_change]

Hi all,
Lately notice that when taking foot off the gas and car drops down in revs and slows down as approaching traffic lights, baring in mind my car is an auto, starting to get a steady, very subtle rumble in the car, very steady but definitely there. This is *before* brake pedal pressed as engine drops to about 12-1500 rpm.

It doesn't stall and engine revs never drop below idle but just feels like somethings not quite right. Engine is fine accelerating and at steady speeds. Cruises along fine. Starts fine. Tyres fine (although coincidentally this started not long after two new rears were fitted)

Whaddayreckon?

[jamie_z4]

cv joint?

[ph001]

Sounds drive train related (I assume if you rev the car in neutral you don't hear it??) but could be lots of things so lets start with stuff that is easy (and free) to check.

1) Wheel bearings. Jack the car up until the wheel is off the ground. Grab the tyre top and bottom and try to wobble the wheel (i.e. push with one hand and pull with the other). Any movement at all would suggest worn bearings.

2) CV joint - wouldn't expect this to cause rumbling but a visual check under the car to make sure CV boot not split is easy enough. A good test is to get the car on full steering lock in a car park and drive around in slow circles in each direction. Worn CV joint usually knock quite loudly when turning.

Other things perhaps like worn diff, prop, gearbox etc are harder to check. Perhaps a fluid change would be worthwhile.

Are your tyres getting towards end of life? I've known unevenly worn tyres to cause similar issues (actually one had a flat spot where it had scrubbed badly).

[jamie_z4]

2) CV joint - wouldn't expect this to cause rumbling but a visual check under the car to make sure CV boot not split is easy enough. A good test is to get the car on full steering lock in a car park and drive around in slow circles in each direction. Worn CV joint usually knock quite loudly when turning.

the inner one causes rumbling and vibrations when coming off throttle, ive had it myself. Outer one causes clicking.

[ph001]

2) CV joint - wouldn't expect this to cause rumbling but a visual check under the car to make sure CV boot not split is easy enough. A good test is to get the car on full steering lock in a car park and drive around in slow circles in each direction. Worn CV joint usually knock quite loudly when turning.

the inner one causes rumbling and vibrations when coming off throttle, ive had it myself. Outer one causes clicking.

Good shout! I was just referring to the outer CV joints but of course there are inner ones also.

[ph001]

And also... not had my coffee this morning! Outer CV joints knocking on full lock doesn't really apply to rear wheel drive cars Doh.

[jamie_z4]

And also... not had my coffee this morning! Outer CV joints knocking on full lock doesn't really apply to rear wheel drive cars Doh.

Well I didn't notice that error either so I need more coffee

[MrPT]

If you only had the rears changed recently, I wouldn't rule out an imbalance that you can only feel at a certain speed. Unusual for it to only be when slowing down or at low speeds though, so if you can rule out any obvious mechanical issues first, that would be the best approach.

Another thing to consider, although again not particularly likely, is whether you have pitched the back of the car up a little with the fresh tyres (>1cm if the old ones were worn) and this could cause unnoticeable rubbing to become noticeable rubbing etc.

[patriot66]

You say it is an auto. Could it be gearbox related ? I used to get some odd noises/frequencies in the car at certain speeds on the old tyres that were on it when I bought it. Changed to non-runflat Avon ZZ5s and it went. Also I recently changed diff and gearbox oil and greased the diff...much quieter now.