Hum from transmission tunnel area?

[Spittybug]

While fuel pump gets my vote (see post #2), a couple of posts in this thread have suggested the secondary shaft nut in the transmission since this is an early VIN. While I DO AGREE that you should separately address that, I don't believe it is the source of your noise.

Inside the transmission case lie 2 parallel shafts loaded up with gears that get meshed together to give the necessary combinations of speeds. These two shafts run end to end and and each is held in place with a big nut on a threaded end. Well, someone in the Renault(?) plant got lazy and on a bunch of the early VINs didn't do the final step when tightening the nut down. He was supposed to hammer the nut's collar against a small flat on the shaft, thus preventing the nut from ever backing off. By not doing so, he has allowed many of these nuts to ever so slowly start spinning off with normal use of the transmission. Soon the shaft gets a little bit of play, for and aft. I believe this contributes to the symptoms of making it harder to shift into second gear as the little bit of play misaligns things a bit and the weak roll pin gets extra stress on it. Ultimately what happens is the nut starts carving a groove in the back part of the relatively soft aluminum cover and one good shift later, the shaft moves enough to crack the case open. Whether related or not (I'm sure it is), also check springs that may get chewed up as a result of things not quite being aligned correctly any more. Ask me how I know this..


So, concentrate on the fuel pump/rubber sound boot/accumulator, but pay very close attention to any change in your shifter feel. Whether you go in proactively just for that or take the opportunity to change the clutch, perhaps add drive shaft sleeves or whatever else needs doing under there, I would plan on doing that for peace of mind.

[guss23]

Humming? Is it a tune you recognize?

[Daniel81DMC]

Yikes! I never imagined that it might be the gears; thanks for the tip on that one. My mechanical expertise sort of precludes me from getting in there and checking this, but my mechanic may be brave enough to have a look down the road.

I really am leaning towards fuel pump, though - the noise is particularly noticeable up front, under the hood, nearest to the windshield, which is almost exactly where the fuel pump is located - I'm pissed off because I replaced the thing no longer than four years ago, after the original of a quarter-century died.

[sdg3205]

With regards to the 5th gear nut, if you pressed the clutch all the way in would it stop?

Also, would it change in speed/pitch if you revved the engine?

[Daniel81DMC]

With regards to the 5th gear nut, if you pressed the clutch all the way in would it stop?

Also, would it change in speed/pitch if you revved the engine?

Good question - I don't think it changes, and heck, you were there the other night - I'd find it odd that the transmission would make audible noises (strong ones at that) as far forward as the trunk area.

I'll see if I can have a good listen tommorrow; car is going to my mechanic's to have the e-brake cable replaced (one snapped).

[DMC5180]

With regards to the 5th gear nut, if you pressed the clutch all the way in would it stop?

Also, would it change in speed/pitch if you revved the engine?

Yes, eventually the rotating parts would stop turning. Even better if you also put it in gear after depressing the clutch this is guaranteed to stop internal rotation.

I'm 99.9% sure it's his fuel pump. They don't build them like they used too. He also did not say what level the fuel was at in the tank. A Low fuel condition with hot fuel (and possibly a collapsing pickup hose) can cause cavitation of the pump which will get very noisy. (transmitted up the tunnel area) This is usually temporary, if the car is allowed to sit over night then restarted and the sound is greatly reduced this is likely is a contributing factor.

What was the reason the pump was changed 4 years ago? OEM pump failure or just because you were afraid it might and figured NEW would be better. If the old one still worked, did you save it as a back up spare?

[Daniel81DMC]

Heya!

Fuel level in the tank is low-ish - a little over 1/4 tank. The sound is definitely quieter when the car is cool and gets louder when the car is warm and has been running awhile.

OEM pump failure is the reason I swapped the last one - it was REALLY loud, particularly on hot summer days. Replacing it made the sound go away, of course, and I was worried that the pump was just going to up and die if I didn't replace it. I'm not one to replace parts unless they really need it. This is not because I am cheap, but rather out of having no desire to waste money on things that are perfectly functional!

[Delorean Industries]

Heya!

Fuel level in the tank is low-ish - a little over 1/4 tank. The sound is definitely quieter when the car is cool and gets louder when the car is warm and has been running awhile.

OEM pump failure is the reason I swapped the last one - it was REALLY loud, particularly on hot summer days. Replacing it made the sound go away, of course, and I was worried that the pump was just going to up and die if I didn't replace it. I'm not one to replace parts unless they really need it. This is not because I am cheap, but rather out of having no desire to waste money on things that are perfectly functional!

The aftermarket pumps available from vendors including myself have been known to be rather noisy during operation from time to time. It is strange because I can pull one brand new and install another one and its completely quite. I have very rarely seen a loud aftermarket pump fail even under heavy use. My daily driver makes noise but hasn't quit after 30k in almost two years of continuous use. The weak link now is electrical connections and if you actually have 12v + a the pump. Low voltage will cause early pump failure and a loud pump. Even though it is proven and I believe even by Bosch that the pump can run at a sub 12v status I think it kills the life of the unit. Also having it completely submerged in fuel matters too. I think keeping the fuel level up is important during extended use to transfer heat from the pump.

[Daniel81DMC]

I was looking at SpecialTAuto for fuel pumps, and they seem to have... several different kinds, some with extra bits (i.e. lines, clamps, etc.) and some without.

I really have no idea which pump I would want. Anyone have a suggestion? By the way, thank you to all of you for being so helpful, it's a real pleasure to be around such a great community.