Odd engine sound

[JimT]

So this seems like an odd one to me, maybe someone on here has some insight.

2012 CM2250 Peterbilt 587.

This morning I noticed that when I apply the service brakes (regular air drum foundation brakes) the engine sound will change. This is stopped and at idle. It sounds similar to the change in engine sound during a high pressure fuel pump test. Or if your stereo minded, like someone increased the bass a little. It only happens when operating the service brake, regardless if the parking brake is set or not. It's not due to the activation of the brake lights as it takes actual application air pressure to occur and goes away immediately upon pressure release.

I don't see how air pressure might have anything to do with what I'm hearing, but I'm no expert here. I do wonder if maybe it's an increase in electrical load caused by the ABS system, though I don't see any voltage drop. I also don't see or hear any loss of air pressure during brake application.

It could be just normal and I'm only hearing it now, but I've had this tractor and trailer for 8 years and I'd think I would have noticed it sooner.

Otherwise everything else appears to be functioning normally. So do I have an actual issue to investigate or am I just hyper sensitive to every little sound?

[hookliftpete]

Does the compressor kick in more frequently than it used to?