Noise in my 340

Take a soaking wet towel and slap it right down on the header pipes at the flange of the headers and under the pipes, bet the noise will change in tone. Just do it for 10 seconds or so, those pipes are HOT.

if the noise change you got an exhaust leak at the header pipes, doesn't matter how tight those little bolts are. what is needed is to pull the header off, clean up both surfaces with a straight edge razor and brake cleaner. Then put some RTV on both sides of the new header gasket. I even used those factory sliver color type gaskets if the ports in the middle are sealed off. They work really good, and I know, it says never to use those gaskets, lol but RTV seals them up.

Don't wiat for the RTV to cure, as soon as its all tight start the motor up but try not to rev it much, you just want it to get hot. After 30--40 seconds turn the engine off and retighten the header bots, you should get a 1/8 of a turn on a few bolts. You might need a few small open end wenches of different brands so you can tighten the bolts well.

Then let it cure for an hour or so and let it rip, fix should last awhile....be sure to use new header bolts so none break