My exhaust seemsm to have got louder recently, and started noticeably popping on downshifts

When i put my hand over the tailpipe it makes absolutely no difference to the engine idle. A lot of gas is obviously escaping from where the tail section is riveted in. Is this normal?
If i drill the rivets out to have a nosey, do i need to put it back together with special rivets (i.e. stainless) or can i use standard ones?
Advice appreciated,
Thanks, Ian