Having fixed the rear wheel bearings - Thank you forum

We now have a noise (thruming best describes it) which seems to be in the area of the drive belt.. Drivers side UK car
How tight should this belt be?
The timing belt itself was changed 4 years ago at 160k and car have done 250k now
The noise is there when the car is stationary and we have looked with the bonnet open but can find no obvious reason

any ideas?