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.