Why do you have a pop sound after revving the engine?

One reason could be after-fire. It's a form of backfiring caused by the engine running too rich. There's excess fuel in the exhaust gasses which ignite in the exhaust. I think it's because the fuel mixture is lagging behind the revs as they lower. As a richer mixture is required at higher RPM's and that mix will be injected into the engine when you rev it, but as you let the throttle out it won't adjust immediately causing this excess of fuel at lower RPM's and some of it won't be burnt in the cylinder, causing the backfire.