What's that popping noise?

[Chad Mac]

Hello eveyone! I've recently acquired a popping noise under the dash that I can only hear in 5th gear under acceleration or at constant speed. It doesn't happen in any other gear and it quits when I get off of the gas in 5th. This popping is a single pop that reoccurs exactly 1 second apart from each other. I can't hear it over the radio and air conditioner but I can hear it well with both of those turned down. It's driving me crazy. I've got a 96 K3500 6.5 TD completely stock w/178K miles on it. It has a NV4500 5spd. I had a new clutch put in approximately 20K ago and rebuilt the entire front end approximately 5K miles ago. The turbo was rebuilt about 35K miles ago. New FSD approximately 8K miles ago. Also, since the new FSD it has run great except for 2-3 times when it has stalled on me after being shut down for 1 hour. It does start back up after about 2 minutes of cranking but . . . any ideas there also? Any suggestions are appreciated. So far, I have solved all my problems through this site thanks to your help.

Thanks,
Chad Mac

[DmaxMaverick]

It could be a number of things with those miles. Have you had your motor/tranny mounts changed? Over time, service on the engine/components gets things under the hood "tweaked". Injector lines, tranny dipstick tube (you don't have one of those), oil dipstick tube (you do have one of those), heater lines, A/C lines, cables, wire harnesses, exhaust pipe, downpipe, turbo oil line, etc.

If you are hearing the noise only in high torque situations, like accellerating and running loaded in OD, it is likely something is "rocking" into something else with the pulse of the motor, like the firewall or floor pan. It could be a HVAC door actuator cycling, but unlikely since it only happens under high torque.

Get under the hood and look for chafing marks. I could be as simple as a battery cable that as become "unhung".

If anything is in contact with an injector line, it will tick with the pulse of the injection time, and can get quite aplified.

Good luck.

[charliepeterson]

Sometimes the heater feed pipe can make contact with and injector line causing a noise like this. A little tweaking might make it go away.

[rjschoolcraft]

If it only happens in fifth gear, then it could be a broken tooth on the overdrive gear set. Does the frequency change with speed?

[Chad Mac]

Thanks for your replies! Yes, only in 5th gear. No noise at all in gears 1-4. The frequency of the pop is exactly one second apart at 50 mph. It does increase with speed but only slightly. The frequency at 65 mph is about 7/8 of a second apart. I was thinking something in the driveline also but all those parts are spinning faster than one revolution per second at 50-65 mph. Can you elaborate on that missing tooth? I'm going to also look at the first two replies to make sure nothing is leaning against an injector line and that everything is tight. Any other ideas are still welcome as I will continue to look for it and keep you posted. It's driving me crazy!

Thanks,
Chad Mac