Hey guys I’m new here. I’ll do an introduction right after this lol.

ive had this problem with my GFs XJ where once it’s warmed up there is an insanely loud whistle on deceleration. Press the throttle and it goes away until you let off. If you let it idle for a little it goes away until you start driving again. I checked all the vacuum lines and those are fine. I soaked the engine in starter fluid to see if maybe there was a small leak somehwere that I can’t find and nothing. My next step was intake manifold gasket... any advice will be much appreciated, thanks guys!