HacGyver

i have been getting a weird knocking type sound at roughly 2800-3500 rpm only.
the noise only comes as i accelerate to 2800-3500. NOT before , and NOT past. it does it in second gear, third gear, and forth gear...so its there. and if i stay in that range the sound goes away. another odd thing is in the early morning when it is cold out , the sound does not come , but in the afternoon when it is warm the sound is there.
does anybody else experience this?
does it sound like the valves? engine? anything else...
what do you guys think it is??

i have taken it to the dealer, and they said it was the engine mounts, so they replaced them, but the sound is still there, and they say they cannot reproduce the sound...maybe if they tried listening , instead of trying to look good in my car they could hear it!!!

basic info on my car...
2002 tl type-s
i only put premium gas in the car since i bought it new.
(been through 4 tank fulls , all different places , and sound is still there.)
only put synthetic mobil 1 oil in it since new. (5w-30)
have 64,000 miles on it.
any other questions??

some people have reccomended on changing the oil ,, i changed it 2 months ago , but i am going to change it again this weekend! any reccomendations? should i change maybe to 10w-30? or 5w-20?

any other reccomendations would be greatly appreciated!!!

thanks