Saginaw tranny "gear rattle"?

A friend of mine bought a 56 Chevy PU,pretty decent truck,283 with a Saginaw 4 speed transmission.He complained the tranny is noisy and has slight rattle sound to it.I took it for a ride and sure enough the tranny makes a rattle sound,not real loud but you can hear it.It's not a regular sound like a bad gear tooth,kinda random,jingly,heard more in second gear when accelerating or maintaining a constant speed.Noise appears to go away when coasting or if you push in the clutch or kick the tranny in neutral when running down the road.When stopped,with the engine running,the tranny is very quiet,clutch in or out.
So...with nothing better to do we pulled the tranny,threw it on the bench.I'm no expert but have had worked on many Saginaws and they all worked great afterwards,more or less,lol.
The tranny was a virgin,appeared to never had been disassembled.The gears looked great,especially the syncro teeth on the gears and sliding syncro hubs.The little teeth were sharp and nor all worn and rounded off like many are.The internal magnet wasn't all full of filings like a metallic mushroom.The gears fit well on the main shaft,all snap rings in place,the end of the mainshaft that fits into the clutch gear was nice.Same for the cluster gear and shaft.The front bearing was slightly worn ,I had a new one and replaced it.The pilot bearing was in good condition,the clutch looked brand new.The tranny rear bushing looked ok, I replaced it with a new one,the driveshaft yoke fit is good.The U-joints are smooth and tight.
Once assembled,The gear mesh between between the main shaft and cluster seemed a little more loose than I remember on other Saginaws.The small reverse gear also appeared a little loose and rocked a bit on it's shaft.There's a bushing for that gear but it looked ok.Wasn't severe and to be truthful I never really looked at this stuff that closely in the past.
Put the tranny back in,90 weight gear oil.makes the exact same noise,the slight random rattle sound.It shifts great,it's smooth in all gears,doesn't pop out of gear when ya back off the gas at hi rpm's. I beat on it through the gears several times,no problem at all.I told him to run and not worry.
This tranny has the 3.11 first gear.Many Saginaws have a thin sheetmetal gear, spring loaded ,on the front of the cluster gear.I believe this is to deaded so called "Gear Rattle". This tranny doesn't have that and maybe the tolerances are a bit on the loose side causing "gear rattle"?
What do you think?

 

Ever dial in the bellhousing to crank center? One symptom of misalignment is rattling. Truck bell with a car input retainer means you can't align em. Good luck

 

This truck bellhousing has the 4-5/8 center hole identical to cars.My limited knowledge says GM trucks used the bigger bellhousing hole on light trucks around 1968 and not on 3 speeds.
Did I throw a dial indicator on the bellhousing?that would be a no,I didn't think about then.Now that you mention it,it might be something to check out.

 

Check for loose stone guard on the rear end pinion shaft. They can jingle like that, and the sound comes up the driveshaft to the stick. Or if there's a two-piece driveshaft, the guard on the center bearing can also do it.

 

Yeah,that's what it sounds like.About the first thing we checked out.It's tight and spinning true,not rubbing on the housing.

 

Some of the old guys used to replace the tailshaft bushing without removing the
tailshaft housing by just driving a new bushing in and forcing the old bushing into the
tailshaft housing to ride forever more on the tailshaft.
I don't know if your saginaw lends itself to this kind of shade tree procedure or not but
it would be easy to check inside.

 

I had the tranny disassembled twice.I replaced the rear bushing when the tailhousing was on the bench.
I went for a ride today.....the noise is just like a loose part jingling around,like a loose year end yoke shield mentioned above,but it ain't the that.The noise is for sure coming from the tranny.Again,no noise when coasting in gear or neutral,no noise when stopped,clutch in or out.No noise when stopped if the engine is reved up.Seems to quiet down slightly after a driving for 1/2 hour. The new front bearing I installed doesn't have a tin splash shield,so it isn't that.
The tranny shifts real nice,normal even gear whine through the gears,could it be a fucked up shift fork making the noise? I looked at the forks,they appeared fine,but?