Noise with AC on -Edited.

[99Yak]

I have a 1999 k1500 Suburban. When the AC (front) is on there is noise that seems emanate from the deep center of dash. Best description would be a low grind, but maybe whirl or even a buzz. It goes away if you turn the knob to left - off position. Or press the the lighted AC toggle. Or when the vehicle stops.

I assumed it was an issue with the center blend/mode actuator or door, because it sounds like, maybe, nylon gears stripping. But, the noise accelerates with the engine. So how would acceleration relate to the actuator?

Any thoughts?

One more piece of information: The mode (?) doesn’t always change immediately. ( Mode is floor to dash to vent. Correct?).

Edit———————

- Changing the Mode is slow or delayed sometimes. Especially Dash Vents Only. However, the “grind“ continues after the switch/change is complete.

- When the A/C toggle on the control switch is toggled off. The noise goes away. Mode will still change without A/C on and without a grind.

- The grind accelerates with RPMs regardless of gear, including Park.

Question: is the ANYTHING inside the fire wall that changes with acceleration (less gauges)?

Question: What happens in the A/C system from the toggle on, when it’s toggled to A/C.

I know this is headed for taking the passenger seat out and sticking my big fat melon under the dash while some one drives - but I thought I might take a shot asking with more clarity.

 

[Levman74]

Almost sounds like it could be RF noise coming through the radio

 

[99Yak]

Almost sounds like it could be RF noise coming through the radio

Right, it’s similar in the way it behaves. But, I get it with radio off and it’s not coming from speakers.

Thanks for taking a shot at it. I hoped it was more common, but the lack of responses makes feel better - because I’m stumped Too.

 

[Eveready]

I wonder if the bearings aren't going in the blower motor? The noise could be the bearing itself or maybe it is wiggling enough that the fan is lightly touching the housing. The motor could vary speeds with engine speed especially if the battery is a little weak and the system is depending on the alternator.

A good test for that is at night with the lights on up against your garage wall. If the the lights get brighter from idle to higher engine speeds the battery may be getting a bit past it, even if it still works

 

[99Yak]

I wonder if the bearings aren't going in the blower motor? The noise could be the bearing itself or maybe it is wiggling enough that the fan is lightly touching the housing. The motor could vary speeds with engine speed especially if the battery is a little weak and the system is depending on the alternator.

A good test for that is at night with the lights on up against your garage wall. If the the lights get brighter from idle to higher engine speeds the battery may be getting a bit past it, even if it still works

Yep. Logical. Then It should make same noise with the fan on
regardless of a/c - but it doesn’t. The fan bearing doesn’t care about a/c, right?

I will do the light test. But this is not a recent issue. It has spanned a rebuilt alternator and at least one battery. It seems to be getting louder which would point to an actuator but that is negated by RPMs. OR, it could be that I’m becoming more easily annoyed

I am also considering the vibration angle:

When the compressor clutch is engage, some loose component in the a/c system, maybe a missing clip, transfers (or creates) a vibration inside the firewall. This would explain engine RPMs and only A/C.

Thanks!

 

[letitsnow]

Is your AC line sitting against the firewall - I've had that transmit the compressor noise into the cab before. I fixed it by adding foam insulation where the line touches the firewall.

 

[Scott Davison]

I've got the same growl out of mine when the A/C compressor is on. I have a small leak to diagnose anyway so I'll let you know what I find.