Question About My Exhaust

I have a weird exhaust leak kind of sound that sometimes comes out the back end of my car. The previous owner had a set up of 2 1/2 inch pipe and then just 2 inch pipe in the back where the mufflers were deleted. I took it to the muffler shop today and the guy said the sound was because of pressure being build up. He said I should have 2 1/2 inch pipe all the way to the back. So two new black exhaust tips (which looks nice) were installed along with the 2 1/2 inch add on. He turned it on and it sounded very nice. But then after about 2 minutes at idle this exhaust leak sound suddenly came about. It's hard to describe but sounds like the car is spitting. The tone still sounds nice and deep but there's this other sound with it. The tips shake a lot and spit water. I turned it on again when I got home and it sounded very nice and smooth. Then I gave it a little test, I turned on the A/C and sure enough the exhaust spitting noise came back. I have no clue what it can be. The guy at the shop checked for leaks but none were found. When I drive around it sounds just fine to me. But it's when it's at idle that it annoys me. Any information helps. Thanks

Good idea. I'll submit asap

View: https://www.youtube.com/watch?v=CH-213qLZMA
here is the sound clip of that annoying exhaust sound at idle. Let me know what you think? thanks

Yea the pressure build up was not the problem. Only access to a lift is through the muffler shop guy I've been taking it to. He's a decent guy but was wrong about the pressure build up. I'll just have to check back with him and see if maybe one of the hangers is loose or I'll ask him if I could stay in the car and maybe give a few revs so he could see if there is any water leaking or carbon residue build up like you said. Thanks for the information! I'll keep looking into this.

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