Howdy all! Looking for some wisdom.

Bought an 01 C5 Auto about 6 months ago. Put about 4k into it, bringing it up to speed. I've listed the services below in case they may have any bearing on the vehicle.

I've since driven the car about 1500 miles. I started noticing that occasionally ( every few weeks it happens - averaging 500 miles per week ) - I will come to a stop at a light, and the car will sit there in idle and exhibit a grinding noise with slight vibration. If I let off the brakes and move forward an inch the grinding/vibration stops until Im stopped again, and then it returns. The moment I take off - she runs smooth as butter. Not throwing any codes.

This is my DD and I want to get in front of anything possible. Id appreciate any and all help.

OIL PRESSURE SENDER
REMOVE INTAKE MANIFOLD, REPLACE OIL PRESSURE
SENDER, REPLACE INTAKE MANIFOLD GASKETS,
INSTALL INTAKE MANIFOLD.

COMPLETE 4 WHEEL DISC BRAKE SERVICE C5/C6
INCLUDES INSTALLATION OF NEW BRAKE PADS &
ROTORS. CLEAN AND LUBRICATE CALIPER SLIDE
MECHANISMS. TORQUE LUG NUTS TO FACTORY
SPECS. ROAD TEST VEHICLE.

REAR DIFFERENTIAL FLUID CHANGE
DRAIN OLD FLUID. INSPECT SEALS INSTALL NEW
FLUID TO PROPER LEVEL.

LUBE, OIL, FILTER
DRAIN OIL, REMOVE & REPLACE OIL FILTER. FILL
CRANKCASE TO VEHICLE SPECS. CHECK AIR FILTER
AND BREATHER FILTER. CHECK ALL FLUID LEVELS.
PREFORM BASIC SAFETY INSPECTION.
FLUID CAPACITY:
ENGINE OIL - VOLUME: 6.50 QTS. (6.2L) - API - SAE
5W-30; W/FILTER - (NOTE: USE SYNTHETIC OIL THAT
MEETS GM STANDARD GM4718M.)

DRAIN AND FLUSH COOLING SYSTEM
INSPECT HOSES AND CLAMPS. PRESSURE TEST
COOLING SYSTEM. FLUSH ENGINE, RADIATOR, AND
HEATER CORE CHECK THERMOSTAT AND ELECTRIC
COOLING FAN OPERATION. BLEED AIR FROM SYSTEM.

POWER STEERING FLUID FLUSH
ROAD TEST FOR POWER STEERING OPERATION.
INSPECT SYSTEM FOR LEAKS. FLUSH POWER
STEERING LINES, PUMP, RACK AND PINION, AND OR
GEAR BOX. INSTALL NEW POWER STEERING FLUID.
ROAD TEST VEHICLE.

BRAKE FLUID FLUSH
CHECK FLUID FOR MOISTURE & CONTAMINATION.
INSPECT ALL HYDRAULIC LINES AND HOSES. INSPECT
CALIPERS, MASTER CYLINDER & CAP. FLUSH ENTIRE
BRAKE SYSTEM INCLUDING MASTER CYLINDER &
CALIPERS UNTI CLEAN.

STABILIZER BAR CONTROL LINK - Remove &
Replace - Front,Both

REMOVE AND REPLACE SWAY BAR END LINKS
STABILIZER BAR CONTROL LINK - Remove &
Replace - Both

REMOVE AND REPLACE SWAY BAR END LINKS
CHANGE AUTOMATIC TRANSMISSION FILTER AND
GASKET

REMOVE AND REPLACE AUTOMATIC TRANSMISSION

FILTER AND GASKET. ADDED FRESH TRANSMISSION
FLUID. FILL FLUID TO VEHICLE SPECS. INSPECT FOR
LEAKS. ROAD TEST VEHICLE.

FUEL FILTER - Remove & Replace - Automatic Trans
In Line Filter

ALTERNATOR DRIVE BELT - Remove & Replace - All
Applicable Models - [Includes: Serpentine and V-Belt
type Belt(s).]

SEAT TRACK REBUILD

 

First get the car in the air and have someone start and idle it on the lift. C5s have many issues with the exhaust system mounts and other things. With the car in the air see if you can reproduce it and zero in on the noise.

Second,, How many miles on the drivetrain?

Vibration / grinding: First thoughts, engine mount (especially on drivers side) If you apply power to the engine with brake ON, does the noise change/;stop?

If you apply a bunch of throttle with the car in gear brake applied, does the drivers side of the engine rise up al lot?? Try it. if it does, that mount is BAD!!! Very common! If you replace one, change them BOTH!!

BC

BC

 

Thanks @Bill Curlee - Since its happening so infrequently it'd be super hard to diagnose on lift - but if it gets more frequent I can use that technique for sure!

Next time it does happen however, I'll give it some juice with the brakes on and see it disappears.

$69K on drivetrain

 

Anyone else have any ideas?

 

I know this is an old post but any chance you found an answer? My car has been doing this for a while now. I don't know how all the internals work but it feels like when you come to a stop something isn't "slipping" in the auto like it should and has a kind of sporadic, rough feeling like it wants to push forward just a little bit; roll forward an inch or 2 and it smooths out and feels like normal idle. Sort of like a rough spot on a contact surface somewhere in there.