Humming sound at startup

[LESDL]

Hello again all,

 

Recently my truck has started making a high-pitched "MMMMMMM" sound in the engine compartment, that will last for a few seconds and then stop. It will then do it again a couple of times before ceasing.

 

You can feel a slight vibration if you have your foot on the brake pedal.

 

It sounds like it may be a pump doing its thing, but I don't think it should be making such a noticeable noise. Performance is unchanged.

 

Any ideas or suggestions for things to be looked at? Brake fluid level and all other fluid levels seem ok.

 

Thanks in advance.

[ChevyTech007]

Found this bulletin, it might apply to your problem:

 

Subject: Supplemental Brake Assist Noisy and May Run Continuously - kw booster brake condition GMT800 GMT805 GMT806 GMT820 GMT830 JC3 JC4 noise pedal vacuum #PI01501 - (11/12/2003)

 

 

 

Models: .

 

 

 

 

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When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms.

 

Condition/Concern:

Supplemental Brake Assist (SBA) motor is noisy and runs when brake pedal is depressed and may continuously run when brake pedal is released.

 

Recommendation/Instructions:

Use P/N 89040236 to correct this customer concern.

 

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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms.

 

Models:

(03 Chevrolet Silverado, Suburban, Tahoe, Avalanche) and (03 GMC Sierra, Yukon, Yukon XL) and (03 Cadillac Escalade, EXT 2-wheel drive)

[LESDL]

Found this bulletin, it might apply to your problem:

 

Subject: Supplemental Brake Assist Noisy and May Run Continuously - kw booster brake condition GMT800 GMT805 GMT806 GMT820 GMT830 JC3 JC4 noise pedal vacuum #PI01501 - (11/12/2003)

 

 

 

 

Thank you! I will keep this info handy for when I make it to the dealer.