Weird sound after shutting off car. Headers possibly?

we just installed my heads this past weekend. we also removed the smog bracket along the back of the heads. right now i have the holes plugged with spark plugs.

when you shut the car off, it sounds like a damn trumpet. the pressure is being relieved and making the noise. could this possibly be my headers or more then likely the smog bolts?

i cut the ceramic part of the plug off with a hacksaw temporarly, could they be releasing pressure? i would have normal bolts in there, except for i cant get any. i went to a bunch of stores who all said try the local bolt store. they can only get them on special order. which is a $20 box for me. so i want to make sure its not something else first.

thanks for any opinions

i think there might be a misunderstanding. the trumpet **** going off is normal? it never did it before.

i actually tried to pin point it more today. the car is too hot to put your hand down and feel for the air, but after shutting it off i put my head down there some. it almost sounds like its coming from around my power steering.

wow ive never heard of it doing that normally before.

thanks for the input. ill check it out after i can get it up to temp again