Low quiet rumble/grumble from underneath at speed?

Hello Everbody,

Did quite a bit of work inside and under the car this weekend.

Replaced the propshafts with a set with brand new UJs.

Replaced underbody heat shields with new ones.

Changed diff oil.

Swapped tyres over to opposite corners.

Also new headlining, and reupholstered sunroof sunshade.

It all went pretty well, but now I have a low down, quiet rumble from the rear at 70mph. Goes away when I lift off the accelerator, and I can't hear it at slower speeds.

It's not a vibration or a judder. It's like the car is doing that Mongolian Throat Singing as you drive along.

I've changed so much I'm not sure what might be causing it!

Do new UJs need to loosen up or run in? They were not tight, but they also weren't floppy like the old ones.

I was pretty careful to mark up the flanges and prop shafts before taking them off, but I might have got the rear one on wrong. Would getting one end 180 degrees out make that much difference?

Lastly, whilst the new headlining is lovely, having material on the sun shade (it's been bare plastic for years) has made the interior much quieter. It used to rattle a lot so perhaps I never noticed the rumble before. Might it be a wheel bearing? And what it the test for that if it is. Rock at 12 and 6, 3 and 9?

No one? ****.

I've never had to try and mend it without any help before!

You did put enough diff oil back in didn't your?

Could be tyres if you've swapped them. Just had a new set on the back of mine & slight noise has gone... was thinking my rear diff was going....again.

I would swap the tyres back as a first point of elimination, tyres do scrub unevenly and can become matched to that corner, had it on my old saab when rotating wheels.

Thanks all.

I'm sure the diff was fully re-filled, but I'll re-check it. You never know.

I think tyres is the most likely culprit. I'll double check the pressures too.

martynv8 said:

Could be tyres if you've swapped them. Just had a new set on the back of mine & slight noise has gone... was thinking my rear diff was going....again.

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+1. I hope the tyres are not unidirectional as you swapped diagonally. Tyres can certainly sing after a swap round.

Sorry for the delay MrG....saw your post this morning and then got pulled out of the office before I could finish my coffee.....and it is still sitting on my desk now!

Not like us to be so tardy in response, esp. to a helpfyul chap such as yourself too

You're starting to get jittery! Probably nothing more than tyres. Everytime I go out I'm listening for something not right.

One of those lovely new tyres of mine is now fecked. I think she must have driven over canyon to get a crack that large in a new tyre.

I'll do some investigation and report back. Might have to wait though, I'm camping at the weekend.