Weird booming thrum noise

[Diesel Fionn]

Swapping the wheels isn't a bad idea as I suspect one of the wheels is out as it was missing balance weights on the outside rim. However my garage that i bought the car from will probably balance the wheels free gratis so I'll do that first.

The model I have is full time 4wd so I can' t disengage all wheel drive. Alll I have is a high low ratio selecter as its an estate model. My identical model saloon didn't have the high low selector. I'll report back once the wheels are balanced.

[Laird_Scooby]

Just a thought - are all 4 tyres the same make and tread pattern and same tread depth across the axles? If one tyre is worn on one side of an axle this can cause an imbalance in the diff as the rolling radius will be slightly different.

[Diesel Fionn]

Thanks for the extra tip, I'll check the tyre levels but they are in good nick and very good brands.

[Laird_Scooby]

No worries, i might be speaking out of turn here but i'm led to believe that the brand/tread pattern matching on the same axle (and preferably all 4 wheels) is more important than the quality of the tyres in the first place. This is based on what a neighbour (who runs an 05 Volvo XC70 AWD) told me he was told by the Volvo dealer. Whether the same would apply to your Subaru i'm not 100% sure but given the theory that all 4 have to turn at the same speed and give the same grip etc i would think so. Presumably along the same lines, if one wheel was out of balance then it could potentially put the whole axle out of balance.

[Diesel Fionn]

Having looked at the tyres today while chekcing the tyre pressurs only the front offside tyre looks less than perfect. It's still good but I think I may need to get the tracking checked soon.
I gave a lift to my garage man who sold me the car today and he noticed the vibration immediately, he reckons it's definitly driveshaft related, possibly a propshaft joint.
So I'll get this checked out soon and will report back soon.