This is a trough question.
The Top gun modulator - if you hear a "snap" - it may not be from the modulator - it may be from the Audio chip "mute" is not quick enough.
How do you determine that?
Well, the Top Gun Modulator - it needs clean power - so if he simply "overlayed" the part on top of the parts that are original and interfaced with...I can only imagine the TGM unit may be asking for more power than the line it's powered by - is not sufficient - meaning the power feed can't be thru a resistor - it can delay the action - either or, the delay in the mute line on the Audio Chip or the quickness of the TGM showing instability during power up - there's your quandary,
So, the TGM usually gets hooked up to the 12V Battery in feed. - and there is a separate switch to turn it on or off - to bypass - but if the installer put the TGM in the 8 volt line - this may be that problem - low-voltage - nothing big, but if he's set up the TGM to switch on in TX mode, versus all the time on and you need to switch it off (Bypass) - there is a delay in the power up - causing the "crunch" or snap.
IF the unit is on all the time, it needs 12 Volts constant to work. IF it's set up for 8 volts, may or may not work "operate" quickly enough - causing you to hear the noise.
Else, if the radio is old, there may be a cap in the Mute line that is taking it's time to discharge, that may be C180 or C181...if both caps are installed - remove the larger value of the two - leave the other in place.
In the above graphic, the Yellow arrows show two caps, C180 is the Oblong One - which is Tantalum - if that has gone bad, that can delay the switchover, hence the snap - see if removing that cap quiets that noise...If it gets worse, then change the cap to a simpler 2.2uF to help with the noise spike of the switching..
You cannot remove all the capacitance for it's part of the Feed for the NFB Equalizer - but Tantalums as they age show their age quicker than typical Electrolytic. So if you feel there is a quality issue, just replace C180 with a Fresh Tantalum of 2.2uF 16~35V rated - you should be ok.
The "bobble" of the RF meter may indicate this condition - you may have had it for a long time (low-frequency "swoosh") - check the soldering around that area of C180 and C181 - make sure it's clean - reheating won't hurt to reflow solder in that area.