OT - Am radio problem

Check aerial connection - especially make sure your antenna is making a good earth to the body of your vehicle. The symptons you describe (lousy when moving) are typical of an antenna base not making a good earth connection to vehicle.

Might even need an earth to go from your radio to the car body.

Is the noise a sharp repetitive crack at ignition rate, or general hash (alternator)? Spark ignition would be present at idle, alternator noise can come on once the revs are raised.
Do you have a diode-type battery combiner for dual batteries? these sometimes cause commutating noise from the alternator (a noisy whine, increasing in pitch with revs). A capacitor (0.5uf) across each diode can fix this.
Check that the radio is not finding its DC earth via the antenna lead or the speaker leads; ie, make sure it's properly earthed fairly close to where it's mounted.
An in-line filter for the DC feed to the radio can sometimes work wonders stopping noise coming back thru the DC feed.
Gerry

Thanks for the tips. I know the headunit is properly earthed but I'll check the antenna earthing tonight. If that's not the problem I'll try the capacitor across the dual-battery unit and the in-line filter on the DC-in.

Thanks for your help

Have you checked the instrument cluster voltage regulator? In that model probably a mechanical reed type. Simple to test, liven up the instruments without the engine on and give the dash a good thump. If noise results you know where to aim for, if not at least another one eliminated.

Leigh.