I have a 1998 mustang gt 4.6l v8 5 speed, and one night I was racing my friends camaro, I was putting all it had into it, shifting at red line and going as fast as I could, then I heard a small ticking noise, which then moved to a strong rattling noise, and doesn't change speed when I accelerate, it still has all the same power as it did before, I know its not the clutch it sounds more like its coming from somewhere in the valve cover, when I checked the oil it was about 2.5 quarts low. we added oil and it didnt do anything or make the noise go away..I have no clue what it would be besides maybe a loose rocker arm I will try to make another post with a video link soon.

To go over it all again it was..

-2.5 quarts low on oil
-went from a small ticking noise to a rattle (only at idle and doesn't make the rattling go faster when I give it gas)
-im positive it IS NOT the clutch
-still has all of the same power as before and easily drivable

 

A '98 4.6 is a pushrod engine? Don't think so...

A bad lifter, exhaust leak, spun crank bearing, etc. will all increase along with rpm's...

 

get a video so we can hear what you mean

 

sounds like a main bearing...especially being that low on oil and running it hard. If it stays consistant during all rpms