hum on 29 classic

Probably coming from your power source. To be sure, disconnect the coax from the radio and see if the noise is still present. If it is, an inline power filter should take care of it. Should be able to find one at a truck stop.

If the noise goes away when you disconnect the coax, first thing to do is check all your antenna and coax connections. Make sure everything is tight.

 

I believe that adding a separate radio-chassis-to-vehicle-chassis ground lead would be more likely to *cause* a ground loop, assuming your power leads are currently run straight to the battery.

Since you've found the source of the noise, it is best eliminated (or at least suppressed at the source: the fan motor.

Solder a 0.01 uF ceramic disk cap across the fan motor's power leads with the cap leads no longer rhan what you need to reach between the terminals. If there's only a hot lead to the fan motor and its case mounts to the fender or firewall for its DC return, run the capacitor from the hot terminal (or splice into the wire right before it enters the motor if there's no conventional terminal) and ground the other cap lead with as short a cap lead as possible. Use a disk cap of a couple hundred volts rating to allow for spikes from the alternator and switched loads.

This bypass cap will largely kill the tiny signal that's radiated from the magnetic field that makes the fan turn, and suppress the electrical noise from arcing motor brushes.

It's better to minimize noise at its point of orgin rather than at the point where it becomes a problem, having had your vehicle's wiring harness(es) to allow other noise generators to i teract with.

Once you realize this works, you may find you're adding bypass caps to electric door window motors, windshield wiper motors, rotating beacon lights, and the like.

YMMV, but hope this helps.

73