If the noise gets faster when gassing it at a stop it is most likely lifter noise, I have a little tick. It could also possibly be a bearing that went out on A/C, alternator, etc. It is not your differential it makes the sound at idle.
I guess if your exhaust manifold pulled off it could be vibrating against the block and give a tick? There are a lot of things it can be, just take a stethascope (sp?) and listen to different parts under the hood and see where it is coming from.
2001 Ironman Daily Driver... 3.5" suspension lift (OME912 springs and 1" spring spacer), ball joint flip, 1.5" front diff. drop, 33"x12.5 TrXus MT, 16x10 Eagle Alloy rims, Interceptor, PV muffler, K&N air filter, Alpine Supercharger, Bilstein shocks, and some trimming.
2000 Ironman Project LS-1 VX... very slow progress but someday....... ohhhhh someday......