new problem help figure out

[angelthepig]

just today i've been hearing a knock sound from engine bay. so i thought maybe my pistons snapped or what not. my car runs fine, when i rev my engine in neutral the sound magnifies, louder and faster. but after a 3 hour rest for my car i took her out for a drive and she seems to run fine, except for the fact that when i'm on the first 3 gears i hear it for 1,500 RPM - 2,800 RPM. when i push my engine harder past those rpm the sound goes away. I haven't tried my reverse yet so I'm not sure if its my transmission. i took her out to the freeways, fourth and fifth run fine with no knocks at all.

i was driving my car pretty hard today a few hours before this happened. but when i popped the hood, the knocks sounded like it can directly from the engine.

so anybody know what could be causing this?is this a engine problem or could it be my transmission. I had a transmission swap recently. it was pretty decent except for my 1st, 2nd, &reverse gear synchos were pretty crap