Strange Grinding/Vibrating Noise

[EclipseOwner95]

Ok, well, im not sure if it started doing it when i got the new engine put in, or when my speedometer broke or when but im noticing now that when i decelerate , or drop RPMS in any way weather it b coasting, braking or pushing in the clutch, i hear this strange grnding/vibrating not sure witch one, it sounds like its coming from the drivers side, but then again im driving so i couldnt tell, i have no idea what it has to do with, it dosnt make my car perform any differnt , but it just scares and anoys me, seens i just got thsi new ( 55k ) motor put in my car, then my speed sensor gear falls into the tranny , but im sure thats not it making the noise, but iuno why its doing that, my alt belt is loose and the dick that put my motor in didnt give it back so its stuck loose, someone said that that made there car make that noise, but iuno, give me your input thx

 

[4g63blinker]

I had a similar problem when I forgot to tighten down the lugnuts on my wheels. I guess its a start.