after the engine is hot and i come to a stop light and idle it will mak a loww bum sound it will do it for a little bit and then stop and not come back, it sound like someone with a bass but if no one is around you it kinda ends that theory it sound like its coming from the engine compartment

 

its like bum, bum, bum, not held out only last a second at the most

 

anybody?

 

does anybody have any ideas, i have no idea if this sound means i need to replace something before its too late, or what it means