The answers to your questions:
1. Loosening the PTO belt helps some, but I'm afraid that the belts will start slipping if run this loose. Also, when they are this loose they seems to flop around a lot, and they look like they might be making contact with the belt guides around the main pulley (the one that is attached to the engine) when the deck blades are engaged.
2. The sound is caused by the main drive pulley (attached to the engine crankshaft) making contact with the deck PTO belt. It is like the belt is partially running in the groove of the pulley, and the pulley will grab the belt as friction builds up. There is a brake that is applied against one of the pulleys for the mower deck spindles which keeps the blades from turning. I have adjusted the tension on the brake as well as the PTO belt tension a number of times. Still getting the howl.
3. Choose the last option, or be prepared to face this situation:
She says, "If something happened to me, would you get married again?"
He says, "I suppose."
She says (with a hurt look), "Would you let her live in our house?"
He says (getting exasperated) "I don't know!"
She says "Would you let her use my golf clubs?"
He says "No."
She says "Why not?"
He says "Because she's left-handed."
She says "Oh...."
He says "****..."