I have three issues.

1. There is a hissing noise from the engine compartment, that makes the engine sound louder.
2. There is a rythmic beat as if something is rolling when the engine runs.
3. After a long halt, if I start the engine, it idles as if in full gas for sometime till the engine heats up when idling becomes normal.

And this 2001 gives meagre 21 mpg though I perfomed the 60k service with fuel system cleaning with the dealer.

Firestone says the hiss may be due to a leaking muffler. My friend said it could be PCV hose leak.

Any suggestions what this could be?

 

did you install a short ram intake on it?

 

I know this is one hell of a bump but if people ever have this problem you can do one of 2 things get an air induction cleaning on it which cleans all the gunk out of the throttle body etc. which i did and fixed the problem. Or you can get your IAC (Idle Air Control Valve) removed and cleaned which costs 100+ and get an air induction cleaning also because the air induction cleaning really does not clean out the IAC that good if at all.

 

Were any sensors left unplugged after the installation, whether it be on accident or due to the new intake not having a place for the sensor?

Also, do you have the check engine light on?