grinding noise

I hope that someone may be able to help me out with this one. I have a 99 Tahoe 4dr with 5.7 liter V8. Occaisionally, however it is getting more frequent, when I first start driving I hear a "grinding" noise that seems to be near the middle of the engine compartment but near the firewall. The odd thing is, if I turn the a/c on it seems to go away. I can literally be driving throught the neighborhood and turn the a/c on and off and the noise will do likewise. And generally only between 1200 and 1800 rpms any more revs and the noise goes away. I appreciate any suggestions people may have. Thanks and Merry Christmas.

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.

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



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.
Grind the ring flange standoffs to the marks established in the previous step on both of the flanges.
Install new exhaust pipe seals, P/N 15093974.
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.






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.

Warranty Information
For vehicles repaired under warranty, use:

Labor Operation
Description
Labor Time

L2040
Seal, Exhaust Pipe (One or Both) - Replace (S/T Truck)
0.5 hr

Seal, Exhaust Pipe (One or Both) - Replace (C/K, M/L, G)
0.4 hr

Add
Flange Modification
0.2 hr

Thanks for the information, but oddly the noise goes away when the a/c is on....and it only does it in drive, not in neutral or park.

Try changing the belt, cheap fix. Bad belt can do alot of weird things like your talking about.

Try listening to your anti-lock brake unit under the hood.