4 speed noises .. help!

I have a 340, 4 speed in my 66 cuda. The trans was just been put in within the last few months and immediately I noticed it made noise in neutral. My previous 4 speed (another story for another day) made the same noise so at first I thought that it must be normal. It's kind of a grinding/ gears working kind of noise that goes away when I push the clutch in. My mechanic says " it's a mechanical device and it's gonna make a certain amount of noise." It think that's bull. He basically just acts like there isn't a problem. Everyone I talk to says there should be no noise in neutral. My clutch pedal vibrates in between shifting gears and I do get some vibration in the handle as well. It shifts fine but over all the trans feels like it's wound up pretty tight, if that make sense.

Resently the same mechanic put an open 8 3/4 rearend in the car for me, changed from a 7.25 and now the trans makes a weird low whine when I push hard in first gear. I don't know if it's related to the rearend but it didn't do it before the rear end was put in.

so I have some issues.. I've never owned or driven a 4 speed car before I owned this one so I have nothing to go on but I have a hard time believing Plymouth made transmissions that made "a certain amount of noise"

Honestly I need to drive/ride and/ or hear what a proper 4 speed is supposed to sound like. Or I need to have someone besides my mechanic drive mine and give me an assessment. I'm starting not trust mechanics anymore so I'd rather haves someone that works on their own car check it out.. If anyone in the Los angeles area can help me out , i would be super grateful. I'm in Sherman Oaks but I'll drive the car anywhere within reason.

If any one has suggestions, info or ideas please hit me up.
thanks