Lately I've noticed a slight whining noise on the truck usually around 40 mph or so. Goes away above or below that speed. What could this be? When do wheel bearings usually go bad? I also noticed the other day there was some grey/black liquid slowly dripping from the front of the muffler when the truck was parked. Couldn't really see where it was coming from.

'07 4.6 Supercrew 2wd

Thanks.