I bought a 2003 Nissan 350z Touring ed. 6-Spd. Today

The thing is when i drive i hear a weird and loud (vibrating or rattling) sounds like exhaust shaking hard) noise when im start off from 0.

At some points its louder than others but it only lasts till at most 2000 Rpm.

Also just to be curious. When i shifting say im in 3rd gear at 3500 rpm. and i push clutch in, it stays at 3500 for about a second or two. and then drops to 800 or w.e it is. this is in every gear. And by the way this is the first Z ive test driven. but the problem is This noise started happening about 10 miles after i bought it so it wasnt like this when i test drove it earlier.

HELPP!!! THanks in advanced