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Strange noise underneath the car

When I start the car, it sounds like a knock, but not anywhere near the hood, but underneath. When I step on the gas, it gives a whirling noise for a bit like there is something stuck on the blade of a fan while it is blowing.

I’ve checked under the hood and it sounds fine (normal). Can’t really see the bottom of the car, so not sure what’s with it.

Recent events: Pour a bottle of Gumout Fuel system cleaner at last fillup. Was fine for a week and then when tank was half empty and refill, the noise started to happen. I did notice that possibly the noise started happening after accidentally lowering the rear bumper about a millimeter when testing my rear shocks. So, my question is: Is it the fuel system cleaner, transmission, or just the 87 gas?

2000 Nissan Pathfinder 4x4