gidyuppp

iTrader: (1)

Hello. New poster, however I have a Grand National as well and find that message board to be a great help so hopefully I can get an idea of what is going on with mine here.

I have a 1988 Iroc Conv. 5speed, 305 TBI. The car is 100 percent stock with 52K miles. It runs awesome and I have no troubles with the car at all. However, last week I drove the car for about 1 hour and pulled it in the garage and heard this humming noise coming from under the car near the rear end. The car was hot so I couldnt do much to see what was up and it stopped about 5 mins after I parked it. As a prevention step I services the differential fluid. I drove it a couple more times and no noise. Last night after a car show it was making the same noise again. Same humming in the same area. My best guess is the differential but I cant generalize the noise because of the hot car. It is coming from behind the rear seat area up to the gas tank. Like I said earlier the car drive fantastic and I have no common problems associated with a rear end issue. The noise goes away quicker than I can identify it. Any ideas?

bradley23150

iTrader: (10)

Just turn the key to "run", dont start it. The fuel pump will prime for about 3 seconds, does it sound the same as the noise your hearing? Maybe the pump isnt turning off when it is supposed to.

gidyuppp

iTrader: (1)

Thanks for the suggestion. The noise I hear is much, much louder than the fuel pump. I tried your suggestion just to pay more attention and that 3 second or so noise is very faint. I then drove it about 20 miles and came home and no sound at all. You dont hear it at lights when top is down, only after car is turned totally off. Even when I hear the noise when car is off I will start it up then go down to listen and it goes away. Quite funny and I never heard anything like this. I owned a 1983 Z28 TBI many years ago and owned that for 12 years and never heard that. Thanks for the help

ASE doc

Power antenna perhaps? If the mast becomes stuck or the mast cable breaks, the motor will run on. The relay is under the right side dash and can be unplugged to turn off the motor.

gidyuppp

iTrader: (1)

Thanks ASE. Noise is clearly between the rear seats and fuel tank. Radio is powered off and it still makes the noise on occasions. Had a suggestion that it is the fuel pump and that having a full tank more than not may stop the pump from working harder??? Worth a try. Since I only hear the noice at certain times, I really have not checked my gas levels when hearing it. I know the other day I was under a 1/4 tank. So I filled up. Its the strangest thing. Thanks everyone

ASE doc

Thats a very good point. I was under the impression that the noise was being heard with the ignition off. In tank pumps can get noisy when they are close to failure. If you determine that to be the cause of the noise, you should consider replacing the pump. No guarantee how long it will last once it starts making noise.

Michelle Mahr

Pressure in the gas tank. Evap canister is building pressure. Unscrew the gas cap.

Caniac

i can promise you that it is your fuel pump. i had the same problem.
the noise keeps getting louder and louder. the longer you drive. sometimes it even shutdown on me. but ya by the time you hear the fuel pump while driving, its a lot louder than it sounds when it primes at the beggining

bought the car changed the fuel pump.
new fuel pump was a faulty fuel pump.
changed it again and car runs perfect.

i dont hear that humming at all anymore.

NY87Iroc

iTrader: (1)

This is what I was going to say..Unscrew the gas cap and see what happenes.