M925 Weird vibration/noise at idle.

I bought a M925 last Friday and over the weekend I managed to break to the tach cable. Soon after I noticed a weird vibration/thump sound @ idle. As soon as I give it a little bit of fuel and raise the RPMs by maybe 100 (not exactly sure as tach isnt working, new cable on the way) the noise completely goes away and stays away until the revs drop back to idle. It also doesn't start when I first start the truck, I have to rev it up a bit and let it drop back down to idle to happen. A part of me wants to think its the tach cable, but I'm almost certain there is no way the cable could make this noise. I had also just filled the tank to full for the first time before the noise appeared.. This noise just sounds out of place. I tried to get a recording on my phone, but it would turn out very well. I've been around the 250s a many times and know they can be a "clunky" engine, but this doesn't sound normal.

Its a model A0, but it does have a turbo/larger injectors and a pyro/boost gauge. Is there a normal issue that may cause this? My perusing through the TM's have really seen that, granted I haven't dug super deep yet.

I will attempt to get a better vid/sound byte of it happening tomorrow.

The air compressor can sound alot like a rod knock at idle after 90 psi

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Do a walk round with a sound device, Like a rod stethoscope or screwdriver etc,,and try various points to try and isolate the sound.

Some noises are normal, listen hard enough and you can hear your mama calling you . If it has good oil pressure and no plumes of smoke out of the oil filler or blow by tube and not hammering from the bottom, run it