Hi guys Help ! Today once arriving home from work, I heard a lot of noise in the engine compartment. I opened her up and couldn't see anything obvious. It clanks loud on start up, then seems to smoothe out a little, like when a water pump is going. I called my mechanic friend, he listened. He took off the serpentine belt, had me start it and no noise was heard . He put it back on, started her up, noise was loud, like a metallic sound. He is not sure, could it be in timing chain area, the damper or something? Or could it be a rod knock sound? The car has been running fine, good power, all that. What do you suggest?

Ok, belt off = no noise, belt on = noise right? That limits the choices of which accessory could be involved, though I have my favorite list of usual suspects.

First, don't worry about the inside of the motor (like rod knock, timing chain, etc) since they would make noise regardless of the accessory drive belt being off the motor.

Start with the idler pulleys and tensioner. They are the simplest, have a single bearing that can go bad and cause them to wobble, rattle, and squeal. Check them over with the belt off. They should rotate smoothly and not wobble (tell-tales of a bad roller bearing...it clicks when it turns and you can feel it as you turn the pulley by hand.)

Next up, A/C compressor clutch, again, turn it by hand and feel for it being loose or wobbling. If it is a A/C compressor, you can use a shorter belt to bypass the compressor until it can be repaired.

Make the same checks to the power steering pump, alternator, and water pump. These are kind of unlikely...both the Power Steering and Water Pump start leaking fluid when the bearings fail...you would find it slung around the engine compartment if those were leaking. Likewise, the alternator would raise it'* ugly head in the form of dead batteries.

The harmonic balancer on the crank nose is a likely canidate...check it over and see if the outer ring has seperated from the hub.

Bless you my son for the reply. We did check all the accessory pumps as you suggested by hand, all seemed ok according to my mechanic buddy. The noise seems to be a liitle worse when engine is cool, its initially loud, then gets a little less so. Giving it gas seems to smoothe it out, a fact my friend thinks is tied to hydraulic oil pressure and why his differential diagnosis includes internal engine problem, i.e. a rod knock. Does this help some more?

BTW, if you do need to short-belt the motor to bypass the AC compressor, try a K060725 belt (6 groove, 73.25" Outside Diameter)...it'* kinda handy to carry one around in the trunk. You really can't bypass the other accessories and still drive the car very far, but loosing the AC compressor isn't critical to operating the car and can be bypasse if needed.

Hmm... Ok, well one item that will rattle like that are the valve lifters...without oil pressure, they rattle until the oil pressure comes up to fill them and remove the slack in the valve-train. In and of itself that'* not an issue, but it could indicate a weak oil pump. (it sounds like a bunch of marbles rolling around in a metal pan)

Does you car have an oil pressure gauge in the instrument panel? If so, what does it read with the engine at curb idle after startup?

If you don't have a gauge, time to plumb one into the system to see what is going on. The place to tap in is on the filter boss..if you don't have a pressure gauge from the factory, you will at least have an oil pressure switch. It can be removed and a pressure gauge installed in it'* place (or use the sensor/switch combo installed on other Bonnies with a gauge in the instrument panel)

Rod knock...it usually does not get any better over time...it starts out and gets progressively worse as the bearing material gets chewed up. I wouldn't assume that until everything else is eliminated.

Something else to look for...a loose bracket or heat shield on the motor. Finding that is a little tough...you need a long screwdriver to use as a steathscope...you are just listening for the rattle and trying to isolate where the noise is comming from.

the oil pressure seems ok, in that it is where it usually has been for the longest time, kind of straight up on the gauge. In the am i am planning to take it to another mechanic for his opinion, will follow up with you , thanks again.

I agree that you should focus on the outside of the engine as opposed to internal problems.
I am leaning toward the crank pulley/ dampener. Mainly because:

andBased on my experience, any of the other belt driven ancillaries that were having bearing or pulley issues would stay loud or get louder as RPMs increased with the belt on instead of their noise decreasing with more RPMs.

To check this, with the belt installed correctly, and the engine off, grab the belt and try to "roll" it one way or the other. If the crank pulley has separated enough to make the noise you are hearing you should be able to "roll" the belt back and forth about 20 or 30 degrees of rotation. If you can't budge the belt at all, your problem is most likely somewhere besides the crank pulley.
A crank pulley costs about $90-$100 US. And they are a breeze to change on these older cars if you have a strong pneumatic impact wrench.
And if that is the problem, then you can usually drive it with a clanking crank pulley for a little while without hurting anything else. It is embarrassingly noisy but you can drive it a bit linger until you can schedule or afford the repair.

If it isn't that then my next guess is the AC clutch or the compressor is kaput.

Probably the harmonic balancer. Same thing happened to my Bonneville. The rubber on the balancer that holds the two pieces solidly together as 1 piece finally got so brittle from engine heat it broke. Every time I started the car it sounded like someone hitting metal on metal. Good Luck. We had to find the right size gear puller to get ours off. Gary Vancouver,Wa.