You don't tell if the noise is mechanical (audible from the unit) or electronic (audible from the speakers) but
If mechanic;
Could be a case of worn motor bearings.
Could also be the mechanics, belt/wheels etc catching on something.
You will have to open it and have a look.
If electronic;
Probably capacitors, the ones in the signal path and around the dolby circuit springs to mind as candidates.
Martin