The answer is that is Rotor Knock. The low vacuum in the manifold caused by the new cam will do this.

 

Is this something I should worry about blownchevy and should I turn up the idle to keep it from knocking or do you have any other idea. Thanks for your reply, I am feeling a bit better about this noise the more research I do. I thought I might have to send it back for service

 

Yes I have turned my idle up a bit but I am running a very mild cam. I've had the maggie on for about 50K miles. Mine only knocks at a cold start until it warms up and the idle smooths out. I wouldn't want it knocking all the time. It cannot be good for it.

 

Thanks for all your replies. I will just turn the idle up a bit and go from there. Thanks again guys

 

Be careful with internet research. Its not the greatest thing in the world for the blower.....try to keep it to a minimum.

 

Have you re-tuned yet for your new cam? After my cam swap, it took me a few weeks of learning HPT, but my blower knock almost completely went away. It sounded REALLY bad at first. I have my idle set around 750 rpm.

 

As stated above, that knock is very common with a cam installed. Basically, at low idle or cold starts, the engine has a lope (or miss). This lope quickly slows down the engine rpm, and through the belt, attempts to slow the blower down too. This creates the knock you are hearing.

Idle tuning can reduce and mostly remove this rotor knock you are hearing. Raising the idle rpms and correcting airflow to compensate for the cam is needed. Ideally you would want a perfectly smooth idle so the blower isn't getting spun at quickly alternating rpm rates.

 

Thanks again. At this time, we are in the process of tuning for the CAM. I will talk with my tune guy and take it from there, I have been really impressed by his tuning abilities (Klugg SS). Right now we are trying to figure out why the truck dies after a hard acceleration with releasing the petal fast. Whether in drive, park, or neutral, if you rev the engine and release the petal, the truck dies. But if you do a controlled cost down of rpm's, the truck idles fine. Damn good job on the tune so far.

 

Got that taken care of.

 

Glad you got it fixed..I went to a rowdier cam and it did the same thing..my idle was at 650ish but had to bump it to 850 to stop the roter knock.