Strange noise from under hood?

[ROAD WARRIOR]

Sometimes when I turn the ignition to the accesory position, I hear a strange humming noise coming from the left side of the engine compartment. It sounds kinda like a modem hooking up and also sounds like it could be a continuation of the lift pump cycle. It will last for about five seconds after the lift pump shuts off and it varies frequency.

Has anybody else ever heard this or am I just hearing things?

Thanks, Dan

------------------
'99 2500 4x4 QC, 5SP, 4. 10, LSD, SLT, DR. PERFORMANCE PHASE III, BANKS 4" EXHAUST(no muffler), BANKS BOOST& EGT GAUGES, SW FUEL PRESSURE GAUGE, BILLET ALUMINUM TURBO INTAKE HORN, AMSOIL AIR FILTER, ROCKFORD FOSGATE AUDIO SYSTEM, DMI QUICK & EASY HITCH, BD 2 LOW KIT, NRA MEMBER

 

[Alan Reagan]

You may have a relay "chattering". Since the lift pump is cycling, pull the relay for it and swap with another in the system to see if this solves your problem. If it does, then replace the relay. Sometimes dirty contacts will cause chatter and sometimes it's just a marginal relay.

After rereading your post, it could be another relay. Pull the top off the PCM and have someone turn the switch on while you watch and see if you can tell if any relays are chattering. Make sure they are all seated by pressing each one firmly into the socket.

[This message has been edited by Alan Reagan (edited 04-25-2001). ]

 

[ROAD WARRIOR]

I have never heard a relay chatter like this. It sounds more like the annoying noise you hear when someone blows across a blade of grass cupped between your hands, or when you let the air out of a balloon by strecthing the neck very tight. It definitely doesn't sound like anything you would expect to hear from an engine compartment. It sounds more like an electronic hum or pneumatic noise.

Thanks, Dan

 

[LSMITH]

I have heard that in mine too! I have no clue what it is, only seem to hear it on mine when the grid heaters are working. It only happens once in a while, seems to happen more often when the truck has sat a few days. If you guys have ever heard an A/C servo whine under a really heavy load, like it is on the edge of an overload,that is what this noise is like. It also varies in pitch sometimes like the load is on and then lowers for a second. I have never heard it for more than about three seconds. I have heard similar noises from pnuematic and hydraulic systems before, so I guess it could be the brake system too. Hope someone knows.

------------------
99 2500 QC 4X4 AUTO SB 24V LARAMIE SLT 3. 5 LSD 285-75-16'S SPA Boost/EGT Everything but leather NRA Member Great Lakes TDR Member

 

[Alan Reagan]

I didn't realize the frequency was that high sounding. I wonder if the grid heaters setting up some type of vibration when you turn to accessory. Like that of a plug in electric heater that will vibrate and rattle when the springs turn red and the metal expansion is trying to keep up with the the sudden heat generation. You may want to make sure the nuts to the grid heater connections are snug and that the plenum bolts through the grid heater are torqued (I believe it's 18 ft lbs). I remember seeing a post where the lower gasket between the grid and the intake was missing. I doubt that's your problem but it is worth checking.

------------------
2001 HO 6 spd 2500, 4. 10 LSD, Towing Package, Electric Brakes, SPA EGT/Boost Gauges, Fog Lights, Real Backup Lights, Molded Running Boards