Congested Engine & Louder Exhaust Note

I have a 1997 Toyota Avalon XLS, & just recently after an oil change I've noticed that the car starts to make a louder somewat rougher exhaust note, also I can smell some gasoline or octane in the cabin when driving.

I tried checking it myself, by opening the hood and giving it some revs to figure out where this problem is coming from..it seems to produced a deeper sound and sounds as if the car is congested.

Any thoughts or help on wat the problem might be?

Have someone completely or partially cover the exhaust pipe at the back of the car. This will force more exhaust out the break in the pipe making it even louder and help you pinpoint the problem.