Squealing in radio

Is the noise

Flat, always the same.

change with turning the fan faster or slower or off?

Change with engine RPM?

Is it in both sides, left and right or only one?

If you play a tape or CD does it still have the noise?

With all motors off, like the fan or any other things like that, the engine not running. does it have the noise?

 

Sounds like an Antenna wire got scraped or the outer cover cut (or kinked)...and it is letting in Radio-interference...RF. The fact it changes with the RPM is very telling!! (if it is happening with AM/FM radio)
This is mostly a guess...but since a heater core and tools has lots of sharp edges...I think it might be worthwhile to reach under there, and try to inspect the cable!!
Now if you get it with XM too, then maybe the radio got wet.....

 

Try grounding the Alternator(mounting bolt down to frame/if its a Cat run the wire from the Alt mount bolt down to the water filter mount), most AM/FM radios have a stud in the back used for mounting support.Use that as a ground connection back to the chassis.
If there isn't a stud then use one of the screws on the radio case.
Use as large a gauge as possible when running ground wires,10ga and up work best for me.

The radio is picking up RF skip from somewhere? the grounding process of elimination
is a PITA

 

Just a guess here, but sounds like a power source issue. the noise is coming in via the power line to the radio. Part of the guess is that when the heater core was relpaced a wire bundle was moved around to allow the tech to work. When this bundle was moved, it put it close to the power source for the radio.

It could be coming in via the antenna lead, good to check it out and see if it is loose or cut. Check at the door hinge as well as under the dash. If it got yanked on, it could have damage in the door.

The computers in the truck may be the source of the interference, not uncommon.
Most have a modules located under the fuse/circuit breaker panel. When the wire bundle was moved to allow for work room, that put it in the right place to broadcast a low level signal that is being picked up by the radio.

My guess is if you get under the dash area on that side, and move the wire bundles around, you may get rid of the noise.

The suggestions to improve ground points for the radio, alternator and some of the motors, like blower fan and wipers are good advice. A number of places sell in-line chokes for these. Not real expensive, about $5.00 each. Most are easy to install. they just plug in to the connectors.