Hmm, may have had a fighting chance with the board I just sent you a pic of.
Now? Not so sure..
The noise can come from many sources, including internal ones.
The problem lies in "overcoming" or "Defeating" that noise at their sources versus just having to run around dealing with it and using the RF gain and Squelch controls. The latter may be your only choice - depending on the chassis you have.
The older Magnums used switches on the front panel and I love those - but were not the best at noise abatement problems - later ones used CPU processor control for turning on and off remotely using toggle functions and special transistors made to work as switches and Audio processing like Noise gating as well as power filtration and RF shunting at the point of contact the switch is supposed to control. Other radio makers like RCI took to this option with some good success - check it out later if you decide to pursue these other avenues of thought and changes...
Yes, changes - not too many people like to know this nor have the ability to do it to their own equipment - but for the most avid and passionate users of radios - they are willing to try just about anything to make their noise floor - more in the basement than say, a tinny air of rushing wind...
IF you turn on your radio and you just have a plain old hiss, this may mean other things that affect the audio chain, not just the receiver is noisy. Although it does lend to know that a noisy noise floor on a receiver that has no antenna connected to it, is a problem child and solutions are not the easiest nor are they for the Feint of heart.
The noise floor of any radio is one of their major selling off or buying one to have it as the best for it - options.
For some radios - it's as simple as changing some resistors in the final audio amp stages that amplify the receivers detected signal some use 2Meg ohm resistors while others use resistors as low at 470 Kilo-ohm - the former value one has a lot more gain, but the latter lower value one offers better bandwidth products to hear your signals as a range of tone versus a mono-tonish compressed result because the dynamic range of the signal is amplified along with the noise embedded in the noise floor and due to it's lack of tone - all scrunches together as one loud "sound" that you are forced to pick out what you really want to hear.
Stopped in to add this...
The above was in a radio the used ST - but it was something they added in to offer the user a better noise floor without having to make a ton of changes to the receiver.
So, before I can even offer a solution - I need to know what I'm up against...
:+> Andy <+: