So by description, the squeal is only when the external speaker is connected...
I hope I have that right...
The easiest fix is to not use the External Speaker - but if you're using a "cubby" type of mounting location, the internal speaker, being covered up, is pretty much useless.
It may not be "RF" that is getting back in...it just could be a misplaced header. That's an assembly issue....
To know that - requires knowing more info - like;
- are you using a EPTDX2912 xx type of board?
- is this a surface SMD type of board now?
- You may need to locate the INTERNAL speakers header - the one that connects the bottom panel speaker - it's to the rear by that power and EXT/PA speaker jacks.
- When they put this together, the internal speaker is hooked up then the put the panels together and throw it in the box
- - if the shop/Store had to do any work, like Frequency Expansion mods
- - put in an echo or perform any work,
- they may not have seated the speaker and the header correctly - the wiring can "pinch" between the panel and the case, forcing a short condition
- - if the correct wire is pinched, it can form the loop and talkback issue you hear.
To answer all this, requires you to open it up, and you might be breaking any seals - Seals that, which if the shop you bought it from sealed it, well ... you lost their ability to fix it on their end.
If that header is not set right, or if there's physical damage back there, this area would "lose" proper ground connection.
It also could depend on the external speaker - and if it uses external power or even has an adjustable volume pot
Let us know how you want to approach this...