Strange noise ??

[scoobydo]

Ok I hope if I throw this out there - I may get the answer so here goes ! (after all you guys are the knowledge !)
1992 SVX with good service history. 106K miles genuine.
After market alloys and tyres. All checked and AOK
JUST rebuilt rear diff - all AOK
JUST fitted NEW rear wheel bearings all AOK
Rear brakes and discs could be better (new ones very soon) but still serviceable so read AOK.

The above was done to try and hunt out a strange noise.......
0 - 40mph we get a whoom whoom whoom that appears to be in tandem with road speed (I wish I could record this so you could hear it !) Above that it is either lost with the sound of the overall speed or it goes - I am not sure which !
The sound would appear to come from the rear left wheel !!! (hence we did that bearing first- and it was done for me by a Subaru expert so has been correctly done)

We are now thinking centre prop bearing ??

And of course thats a pita to get to (new SS exhaust coming as the one currently on wont be able to go back on !!)
There was lift in the diff input (which is now fixed)
The Auto box appears to be fine and works as it should.
Anyone had this ?? or can suggest a possible?

Thanks !