Thanks for all your response.
If I press the gas more, it's hard to say the engine noise significantly increasing. And I have tried to use shoe to block the exhaust pipe, the engine didn't stop. But as you know, I couldn't fully block the exhaust pipe just by my shoe. So I have a couple of questions,
1) Is it big effort to figure out the exact problem if I ask a mechanic to inspect it?
2) Let's say the exhaust pipe leak, is there any hurt except more noise? Usually how much to fix the problem including parts and labour?
Thanks again.