Quick update on this - the rumbling noise is still there and possibly getting a bit louder but not by much. Popped into my local indy garage who I've used before this morning to get them to take a look and quote for the job. He took it out for a spin and is 99% sure its not the bearings but showed me that the tyres are a bit deformed. Basically if you spin the wheel and hold your hand against the tread near the inside of the tyre then you can feel bumps as it spins past your hand. He didn't know what causes this but suggested I change the tyre when it needs doing (got a few '000 miles on it yet) and see if the rumbling noise goes away. That's one less bill to pay and impressed that the guy was honest enough to basically turn down a repair job that doesn't need doing - not all of them would do that I'm sure.
- jeebowhite likes this