Hello everyone,

I am a fairly new 1988 C4 owner. We rescued my wife's uncles C4 when he was put into a nursing home. Car has almost 18,000 original miles on it. Its in really decent shape. I have been busy undoing some the the mechanic hacks that have been done to it over the years. 

One noise I cannot figure out....

When I put the car into drive or reverse, and then it rolls maybe 5-10 feet, I get his motor sounding  hum and the brake pedal slightly depresses towards the floor. That is the only time it does this. The hum is definitely coming from the rear end of the car. I don't think it is the fuel pump, as the pump is really quiet when you just turn on the ignition to on. The brakes are firm and they don't go spongy like I have a vacuum leak.  I have searched through parts manuals and I can't find another motor in the rear end that could be causing this. Anyone had this issue? Thanks in advance!

Hello Bryan

As a guess, it might be the computer doing a check on the antilock brake system. Mine is a 91 and it always does what you describe. Sounds like you have picked up a very low mileage vette, good luck with it.

Yup its the ABS under the storage box behind the drivers seat.

So is this a normal noise? Or do I need to be checking into a brake issue with the abs?

Hi Bryan, 

I have a 30,00 mile 94 that I have owned since it had 13,000 miles. Since the first time I backed it out of my garage and put it in drive and start moving forward it makes a short groan. This happens as I turn and start down the road.  I have traced the noise back to the differential. I was told it is the clutches in the posi unit. I serviced the fluid and put in the GM recommended fluid and additive and to this day the noise remains. I was told it is quite common and harmless. I have a 2:59 gear and have had no other issues with the car. I get no indication that it is from the anti lock brake system from the DIC or the brake pedal. 

Thank you for that information! I think mine is more brake related as my brake pedal slightly depresses at the moment the hum is happening. But I will definitely change my rear end fluid to be safe. Thanks again!