AUDI A4 1.9 TDI SE (1996) - REAR WHEEL HUM

I have an intermittant "hum" coming from the rear nearside of the car. It happens at all speeds although, if I drive really slowly at can be reduced
to a single pulse that seems to time in with the speed of the wheel revolving. It doesn't happen all the time. Any ideas what it could be?
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a bearing no doubt
mine were done a few years back
dj A4 1997
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