Why does my 727 make noise?

The 727 in my 79 LRE has been making noise...really quite soon after it was rebuilt.

It started small, and at first didn't think it was my transmission...it'd occur under very light loads/high vacuum situations, so I actually thought that it was noise from air resonating through my throttle plates...as soon as a little throttle was applied, it would go away, and come back under low load/high vacuum situations again.

After taking my Truck to the Drag Strip though, the sound is unmistakably the transmission. It only happens in 2nd and 3rd gear...but has gotten MUCH louder. Sounds kind of like a loud flat horn...

And again, under light load...when I apply throttle, the noise goes away. If I replicate the same throttle conditions/rpm's in 1st, no noises, load or no load. Trans was "built" with components to race (shift kit, race clutches/bands, high pressure regulator spring, etc)

Transmission otherwise drives just fine...no slipping, good firm shifts.

From A to Z the problems could be a multitude of issues?, first I'd start with checking the converter flex plate, the bolts, any signs of rubbing/scrapping, weights missing from the converter, etc, etc,...new drive shaft? vehicle been lowered, spring/axle/perch relocation, etc?

A "moan", "buzz", "whine" noise is often caused by a rapidly cycling pressure regulator valve, the factory calls it "regulator valve resonance". It's more of an annoyance than a real problem...the recommended cure is to replace the valve body.

Maybe the low / reverse band is adjusted a bit too tight, not fully releasing?

Since it was a second and third gear thing, I thought band might be an issue, but John's description sounds the most accurate and makes sense that the noise depends on which gear it's in too. It does shift fine/no slippage....which fits with "annoying...not serious."

Though this one is VERY annoying since it's VERY loud....

You're positive it's the trans? I have a '01 Durango with a bad pinion bearing and it makes noise when applying light throttle, but quiets way down when I let off. Sounds pretty familiar...

That's a good thought...but 1)the noise definitely doesn't sound like it's coming from the rear 2) Can't find any load/speed conditions when the noise can be repeated in first gear 3) Going into neutral completely quiets it down (I know this changes load conditions so it doesn't entirely rule out bearing) and 4) the noise was present before and after completely rebuilding the rear end.

Cool. Just a thought.