Hi, I Daily drive a 1985 Chevy El Camino with a 305v8 350 Turbohydromatic 3 speed, with around 116,500 miles.

Me and a fellow I know where talking about cars after class, I decided to show him what a real old school engine sounds like so I do a gentle reve like I do, all of a sudden I hear a loud pop sound, I’m assuming a back fire from the carb cause I just started the car 2 minutes ago at that point, I listen to it for a minute check my temp and oil PSI. Everything reads fine and sounds normal, I dive the 6 minute drive home and when I put it in park a hear a just noticing noise from down below, a noise like buzzing from a fly or humming from my transmission pan area. 

I turn the car off let it sit for a sec and slowly shift though all the gears back to park and start the car, the noise went away but after a few minutes it came back but not as loud what could that be.