Ok Dan, first--It is most likely not a cracked exaust manifold. If this were cracked the noise would sound like a single engined prop plane and your engine would run very bad. It may be your alternators' bearing going bad or something of that nature. Here is an easy way to check. Your engine has s serpentine belt on it. Find the belt tention pully and take the tention off the belt. I believe a 3/8 ratchet will fit in the square hole to do this. Now spin the alternator and everything else that will spin with the tention off the belt. Listen for any worn out bearings on all the pullys, alternator, powersteering pump, ac compressor--etc.