This site is amazing!  Your input and suggestions is very much appreciated and I thank you for taking the time to respond.

I will get some cleaner and try that on the controls and FX return jack.

Last night while I was working with the amp.  The volume level would going up and down.  But I could wiggle the input cable around in the jack and get it back to a full volume.  It did seem like once I found a good spot, the noises went away and the amp sounded great.  As long as I did not move the input cable. 

Could a loose input jack be causing the breathing noises?

One last thing.  I do still have the original footswitch, but no longer have the cable.  When I turn the amp on, the amp is already in boost channel.  Last night I took a foot switch from my Fender amp.  Plugged it in the Marshall.  When I clicked the button, the boost light would go off.  But I would not get any sound from the clean channel.   I am not sure if the fender footswitch was not actually putting the amp in to the clean channel, or maybe just turning off the boost channel.

Could it be possible that by not using the original footswitch, the clean channel was not activated?
Is there a way to manually put the amp in to clean mode?

I am very excited about using this amp.  Thanks for everyone's help so far

Amazing, that did fix the issue above.  I know the amp is older and going to have some noises.  But there are a couple noises I wonder if they can be fixed.

1.  When the amp is on, but the guitar's volume is turned off, you can hear noises coming from the amp.  Almost sounds like the amp is breathing.

2.  When you adjust some of the levels (volume, reverb, etc) the amp makes a popping noise.  Only happens when you turn the knobs.

I really appreciate the help received so far.  Even if there no easy fix, just knowing what the issue may be, puts me ahead of the game on getting it repaired.

Recently I was given a early 80's Marshall 5210.  Plugged my guitar and started playing.  I really enjoyed the sound of the amp.  After about 10 minutes, the amp started cracking and making popping noises.  Then the amp went quite.  I was not getting any sound what so ever.  I can turn the amp off and let it set about 20 minutes.  Turn it back on and I get about another 10 minutes of playing time before it does it again.  I am not sure if this is a common issue with the model of amp.  I am looking for some suggestions on what it may be.  As well as if there is any thing I can do to test it.

It was a free amp, but if I can get it working, that would be so great!

Thanks in advance for any suggestions you may have.