What could this be?

[BlackCobra50]

When I run my car hard (from a stop) and shift quick like im racing, when i engage the next gear (ie shift from 1st to 2nd) it sounds like its backfiring... im not sure how to explain it but if i need to ill put a video up of it. anyone know why it would be doing this? :wall:

 

[scarface]

its probably backfiring. may be running a tad rich.