It sounds like a bad connection, burned component or cold solder joint to me. It could possibly be a tube. Figure out where it is comming from. Unplug the Quad from the 50/50. If it goes away, it's in the Quad. If it stays it's in the 50/50. Put your ear up to each speaker or cabinet to see which channel it is comming from (left, right, or both). If the noise is the same in both channels, it's probably in the input/driver section. Next, turn the channel volumes on the 50/50 all the way down. If it goes away, the problem is in the input/driver section of the 50/50. If you still hear it with the 50/50 volume all the way down, it is in the phase inverter or power tube section of the amp.
Also, check each of your tube sockets to make sure they are all clean and properly tensioned. You may need to squeeze the individual pin sockets a little with a pair of needle nose pliers to make sure each is getting proper contact with the tube pins. Work the tubes in and out of the sockets a few times to scrape loose any crud that's in the sockets. If a socket looks burned, it has been arcing and will have to be replaced. If that doesn't do it, start swapping tubes. If that doesn't fix it, you're probably looking at a burned component or cracked/cold solder joint. Open the amp up and closely inspect the circuit board for burned resistors, bulging caps, etc. Cracked or cold solder joints can be fixed by quickly reheating them with a soldering iron and allowing them to slowly cool. Solder joints should be very smooth and shiny. If they are rough, cloudy/dull, or cracked looking, they might cause a poor connection. Make sure the component or wire does not move AT ALL while the solder is cooling. Also NEVER blow on a hot solder joint to make it cool faster. Cold solder joints can be very frustrating to hunt down. When in doubt, take it to a good tech.
