The radio's VCO consists of its discrete components in addition to the evironment that surrounds it. This includes shields and PCB ground planes. When a radio is new and solid everything works great. But over time hardware loosens, mechanical connections become fouled by contaminates or corrosion or wear. When this happens manipulation of a bad junction will cause the VCO to react and jump around slightly. A speaker turned up high enough will vibrate the innards and modulate the VCO. This is detected like a normal FM signal and goes through the audio chain. There is your feedback that causes that screaming howling noise. Of course the squelch needs to be open for this to occur.
I find pads to reduce vibration are pretty much useless. Sometimes a component like a coil will quiet down a little with a dab of hot glue to pot it. The rework kits as mentioned usually are modified grounding straps and fingers and never hurt.
When I get a real tough dog I go all in. I remove the RF board and scrub all grounding lands and case posts with contact cleaner. Same with removeable shields and finger stock etc. Then maybe do the pencil eraser thing to shine things up. I then apply some Stabilent 22 to these mechanical junctions (my secret weapon). Reassemble and make sure everything is secure and tight.
That takes care a lot of these type of issues. By the way, transmit can be affected too. Hold a portable in both hands and transmit. Twist and bend the radio and listen for the crunchies as you torque on it. If it gets bad it can affect your transmit under normal banging around.
