I bought a '99 Tahoe 2dr one year ago from original owner. Has 107,000 miles and is in excellent condition. I've started noticing a very strange noise coming from the engine compartment. I'm a mechanic (back yard) and a machinist, this sound is like nothing I've ever heard before. The only way I can describe the sound is by saying it sounds like the flywheel teeth barely touching some nearby sheetmetal (a fast clicking sound). I know it's not a lifter/rocker or valve clatter, I'm positive. It only does it once in a while and seems to increase with the engine RPM. It will even do it in park if I rev-up the engine but only for a few seconds. If I'm in drive the noise comes and goes sometimes when I first accelerate and sometimes when accelerating at any speed. It's not very loud and it's a sound I've never heard coming from an engine.
5.7 liter/auto/4:10/33X1250X15 tires/hypertech programming. Has all new front end overhaul and new rear gear/bearings.
Any help is appreciated, will try and isolate this weekend and was hoping someone had heard this type of noise before.

Thanks,
Treblig in Tex

I dont know if this will help but I have something similar with my 99 2door Tahoe. Here is a service bulletin I found:
Exhaust System Rumble/Rattle Noise (Replace Exhaust Pipe Seals and Modify Exhaust Pipe Ring Flanges) #01-06-05-006 - (Nov 2, 2001)
Exhaust System Rumble/Rattle Noise (Replace Exhaust Pipe Seals and Modify Exhaust Pipe Ring Flanges)
1996-1998 Chevrolet and GMC C/K Models with 4.3L Engine (VIN W -- RPO L35)

1996-1999 Chevrolet and GMC C/K Models (Old Style) with 5.0L or 5.7L Engine (VINs M, R -- RPOs L30, L31)

2000 Chevrolet and GMC C/K Models (Old Style) with 5.7L Engine (VIN R -- RPO L31)

1996-2002 Chevrolet and GMC G-Vans with 4.3L, 5.0L or 5.7L Engine (VINs W, M, R -- RPOs L35, L30 or L31)

1996-2002 Chevrolet and GMC S/T, M/L Models with 4.3L Engine (VIN W -- RPO L35)

1999-2000 Cadillac Escalade

1996-2002 Oldsmobile Bravada

Condition
Some customers may comment on a rumble/rattle-type noise that is audible in the vehicle passenger compartment when the engine is idling, the transmission is in Drive and the service brakes are applied. High ambient temperatures and A/C compressor cycling may further aggravate this condition.

Cause
This condition may be the result of inadequate clampload on the exhaust pipe seals at the exhaust pipe/manifold joint. The low seal clampload may be the result of the standoffs on either one or both of the exhaust pipe ring flanges contacting the exhaust manifold.

Correction
This condition can be corrected by replacing the existing exhaust pipe seals with new seals, P/N 15093974, and modifying the exhaust pipe ring flanges. The ring flange modification consists of reducing the height of all three of the standoffs on both of the exhaust pipe ring flanges by 2 mm (0.08 in). Follow the procedure below.

1.. Following the Service Manual procedure, lower the exhaust system down enough to access the exhaust pipe ring flanges.
2.. Remove and discard the existing exhaust pipe seals.



3.. With the ring flanges slid down the exhaust pipe, mark each of the three standoffs on both the rings 2 mm (0.08 in) down from the surface that faces the exhaust manifold.
4.. Grind the ring flange standoffs to the marks established in the previous step on both of the flanges.
5.. Install new exhaust pipe seals, P/N 15093974.
6.. Raise the exhaust pipe into position and hand start the ring flange to exhaust manifold fasteners.
Notice: Use the tightening sequence and specifications indicated below when installing fasteners in order to avoid damage to parts and systems. If exhaust system is disconnected at any location other than at the exhaust seal , gasket and exhaust system alignment may be required.





7.. Following the fastener tightening sequence, gradually draw the ring flanges toward the exhaust manifold seal faces, maintaining the ring flanges as parallel to the manifold faces as possible.

Tighten
Tighten the exhaust ring flange to exhaust manifold fasteners to the specification indicated below by vehicle year and model.

Year/Model
Tightening Specification

1996-02 S/T-Truck
53 N·m (39 lb ft)

1996-02 M/L-Van
53 N·m (39 lb ft)

1996-97 C/K-Truck (Old Style)
48 N·m (35 lb ft)

1998-00 C/K-Truck (Old Style)
64 N·m (47 lb ft)

1996-02 G-Van
65 N·m (48 lb ft)


Parts Information
Part Number
Description
Qty

15093974
Seal - Exhaust Manifold Pipe
2


Parts are currently available from GMSPO.


GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.
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Thanks R7,

I'll be sure and check it out. Even if it isn't the exhaust flange you gave me some good information.

Treblig, Tx

Treblig, not an evap solenoid cycling, possible?

it sounds like a cracked exhaust manifold.

A cracked exhaust manifold should make noise continuosly once it's warmed-up. I've noticed that the noise is more pronounced when I put the vent knob in any "windshield" position and it only does sometimes (that's what's throwing me).
Where would the solenoid be located? I do hear valves opening and closing when I put the vent selection knob to the windshield position but I have no idea where the valves/solenoids are located