McIntosh mono amp red light flashes after a couple hours

I’m sure others with significantly more knowledge will follow, but here’s my thoughts. First off, this is not good and needs to be remedied fast. Are you at a moderate or high volume ? Dim the lights and see if a tube LOOKS RED. Switch all the tubes from left side to right side and see if that affects anything. If it switches sides you’ll know it’s a tube. If it’s still the same side have a tech look inside. I don’t have Mac’s, but in my gear I’ve had tubes that let go and didn’t hurt the amp and I’ve had tubes let go and damage the amp. I’ve had resistors on the phase stage fail and cause noise issues. I’ve had bridge rectification get tired and cause noise issues. Good luck and be careful, Mike B. 

If you have a replacement set of tubes, then replace all the tubes (if you don't... you should have a replacement set). Then take your regular set of tubes in to be tested. 

Did you provide the model number of your amp and did I miss that? Also your speakers, listening level info would be helpful. Lastly, did you do the very first thing you should do which is call McIntosh service/support? They're really helpful and know the product best.