Fre3way

I have a 1999 Es 300 with 150k mileage on it. Its always been maintained by a Lexus dealership until 145k. Recently, I've been hearing a little muffling noise when I push the accelerator. It only happens when I push it, the moment I let go, the noise goes away. It sounds like the car exhaust making that noise. Any advice on how to fix this issue would be greatly appreciated.

P.S. Recently I've noticed that when I open my gas tank cap, not the cover, I'm talking about the actual gas cap, it feels like some built up pressure is released. I recently started hearing this too so maybe this has something to do with it.