Whole House Attic Fan stopped working after a loud pop?

My boyfriend installed a new ceiling fan to replace the outdated one that was there - it works just fine, the lights, the fan - but almost every time after it has been turned off it will make a loud pop noise - it can take a while for it to make that noise and its not timmed enough to have someone...

The attic is making noises loud sharpe noises. It happen about 11:00pm and again 7:30 & 8:00am. I noticed the raffters are bowwing. It is hard to tell how much. Also my vent for plumbing, a pvc pipe going through the roof is off center it is toward the top off the roof line and toward the bottom...

I was approaching a stopping point, pushed in the brake/clutch pedal, heard a loud pop and suddenly realized I had no clutch on my sears craftsman riding mower...can someone help ? Thanks!

When I turn it on I hear the engine humming, but blades barely move, I have to open the vents manually to see this, they don't open. Never worked with one of these before, any help would be appreciated. It is mounted above my 2nd story ceiling in an accessible attic. Have no idea what the effort is...

The fan in the attic is not blowing with the a/c on (the compressor will run outside) or if I just turn the fan to on on the thermostat. However, it will run if I turn on the heater. Suggestions? Thanks!