Odd hum/thrum from transmission area at ~30mph

[Neil_]

Hello,

I have recently purchased 3.2 XC90, about 38k on clock on a 56 plate, previous Chelsea tractor I think. It has developed a weird.thrumming noise and feel from what I would say it the bevel.gear area, it becomes.apparent at about 30 - 40mph, then goes away for and comes back but not as bad at about 60mph.

No sensation through steering, I swapped front and back tyres all round in case of some weird alignment issue, no change.

I've also tried knocking the transmission into neutral whilst it it humming at about 35mph, engine revs drop but humming is unchanged so I think it is on drive side of the transmission.

Any ideas? It is pretty low mileage, so surprised anything is showing wear/tear this early.

Not sure how to check all of these but thinking a list could be:
Wheel bearings
CV joints
Transfer box (no visible leaks)
Propahaft - (how and what do I check for?!)

I've got hold off an old little camera I'm thinking about strapping to the bottom of car somewhere to see if I can hear/see vibration on video footage.....just need to find an old.memory card reader...

Any pointers or suggestions much appreciated,

Neil

[TruckbusUK]

Add to your list the air con compressor and pully, it can give similar.
However you dont say if the noise increases if you accelerate, rather than it just coming and going dependent on your speed ... carefully check the tyres again for bulges or uneven wear, finally check the noise when you corner (use a quiet roundabout,) and see if it changes when the weight loads and unloads ... that would suggest wheel bearing ... but they can be a pig to identify.

Re the prop .. check the centre rubber donut/bush see how badly worn it is. plenty of threads, have a read.

Just some suggestions, no mean exhaustive but have fun tracking it down

[Tannaton]

Check the CV joint on the front of the prop where it meets the transfer box....

[Neil_]

Great thanks so much for the suggestions guys. Really want to get it sorted!

Noise doesn't increase with acceleration, more of a 'harmonic' that is reached 30-40mph, goes away a bit until about 60-70mph. Pretty sure not engine speed related. Did try a twists and turns at 30 in an empty car park , don't think it changed it, but was concentrating more on throwing the car around safely! Will find a bigger space!

[Neil_]

Ok, so it has been a while. But a few days off for Xmas and I had a chance to get under the car.... Took the propshaft off, that was fun, but the bad news is that the vibration is still there, but it is much much quieter, I'm guessing the vibrations were passing along to the propshaft somehow which amplified it. The CV joint wasn't great, so I will replace that whilst it's out....

Dark now so leaving for tonight, pretty frustrated!

[ian ferguson]

i had something similar on my D5 i was convinced it was a wheel bearing, but it was so difficult to find, i ended up changing all of them one by one, of course it was the last one

[nickbw898]

Could be worn drive shaft splines or wheel bearing. If CV joint is shot then so is drive shaft.

[Neil_]

Looking around on web I found a few American forums which suggest that the inner CV of the RHS driveshaft often dies.

Wonder if it'll drive with just the left and the prop connected to test?......

Really feels like it is coming from transmission area, so reckon this is the next place to lookj.

[Neil_]

Just in case someone else happens to have the same thing, thrum is solved, it was the driver's side wheel bearing.

Couldn't feel any play in tyre when jacked up, but once drivershaft was out and wheel off I could feel the bearings grinding when turning it by hand. Sorted now

Weird it was such low mileage to go, think SWMBO might have hit something.....