It's about two years that I can hear a clack-clack sound coming from the engine bay: this appears only in a certain rpm window (2000 - 2500 rpm).

This happens only whith a low load: it disappears in decel and acceleration. It's there only when I keep the speed constant, I have to "aim" at it to make it noticeable.

It's much more noticeable when the engine is cold, once warmed up it's much more difficult to identify it between the other "normal" noises.

Without a certain load it's impossible to hear that specific noise, so I have to drive to hear it. At idle I can't spot it, not even using a sthetoscope.

It's not an accessory (I removed the belt).

I disconnected the injectors one by one hoping to identify something, but the noise was always there, same rpm window, same intensity. This is a procedure described in the service manual. They just say to disconnect the coils. For me it was easier to disconnect the injectors.

The engine a 98 internally stock LS1 (85'000 miles), the only mechanical thing I changed is a ATI harmonic balancer (it drives the supercharger).
I use a synthetic 0W40 oil.
No misfire, no codes, no KR, the fuel trims around 0%.
Right now the supercharger is disconnected and the program is 99% stock.

What else can I check before I start taking the engine apart?
What could it be?
Rollers? Pushrods? Lifters? Timing chain?

If it was one of those parts I think I would hear the noise at idle... or not necessary?
Any idea?


Thanks

Stefano

Is the noise on one side of engine or both ?

The LSx engines are not quite, mine is worse when cold and it is simply valve train noise.

You need to pin down where it is coming from but if its on both sides it is anything from the lifters, maybe a flat spot on a lobe of CAM ?

Bearing from Alt or A/C compressor ?

Try a 15W50 oil just to see if any different

Difficult to say from where it's coming from. As I wrote I have to drive around to hear it (it needs some load to be noisy).

I tried with a friend putting some load on the engine while the car wasn't moving (hand brake, playing with cluth and throttle pedals) and I would say the noise is coming from the middle of the intake manifold (?).

I removed both belts to see if it was an accessory, but the noise didn't change.

If it was a bent pushrod would I hear it from idle? Or is that something that comes up at a certain RPM?

I posted the noise here:
http://videos.streetfire.net/video/ec2160b4-470d-4426-b420-ddc71ffb0c0f.htm

(it's from a small camera I put close to a heat extractor of my hood, so you'll hear almost every possible noise, I also have 36 lbs Bosch injectors that are noisier that the stockers).

For whatever reason when I try to run your video it fails so cannot hear the engine.

I'd closely check the valve train as pushrods, weak valve springs.

How can I check the valvesprings without removing them?
Is it just broken - non broken or I need to measure their strenght?

Probably you need the right player for the video (you need the Adobe player). Where can I post it? The file is too big for an email...

Stefano

How old and what type of valve springs ?

I do not have my laptop setup for video since it is too easy to get hacker or virus crap via them.

The valvesprings are the original ones that came in 98, 85'000 miles ago.
Stock and old...

I guess the same style that came in the 97-98 Corvette.

Well then I would suspect they are weak with that kinda mileage on them.
You can change them while heads are still on the car.

You could find some stock LS6 ones fairly cheap or go with the beehive with lighter retainers work well.
At same time you could look at pushrods and make sure they still are true.
I changed the ones on my 1999 C5 and it was worth replacing the old springs and retainers

review these
Springs

and

More springs

Thank you, I'll give it a try.
I'm just not sure if I can replace the springs without removing the engine: in F-Bodies the engine is pretty hidden.

If you use the right valve spring tool you should be able to get to the back springs like this tool which handles both spring at once of a cylinder

Just make sure you use a air compressor to injector air into the sparkplug hole to keep the valves up while doing the spring swap.