1976 Feather Duster A-833 4speed overdrive trouble shoot help

I'm getting a "rattle" like sound coming from my tranny only in Overdrive gear. Anyone experience something like this? Clutch is not the issue as in all other gears there is no issue of noise. Could a shift rod be out of adjustment or possibly the shifter stop bolts are out of adjustment? Just bought the car only driven half dozen times as of yet. Note that coasting in od no noises are apparent but under load throughout the rpm range I'm getting quite the racket. Any Leads as where to start would be muchly appreciated.Figure trying cpl simple diagnoses first would be better then tearing out the trans and replacing it.

1976 Plymouth Feather Duster, 360, 4speed {od} manual, 3:55 Suregrip posi 8 3/4  
76 Feather Duster
360ci, 4speed manual(overdrive unit)
355 posi 8 3/34 rear end
edelbrock high rise aluminum intake 650 cfm carb
Msd 6al digital rev limiter ignition
Flo-tech headers
2 1/2" dual flow master exhaust

you could have a chipped gear..

you could have a burned out gear on the main shaft..

you could have a worn out syncro.. or the sliding blocks have come out..

here is a well done article right here in the magazine that you will want to go thru page by page..

http://www.moparmusclemagazine.com/techarticles/drivetrain/mopp_0110_a_833_aluminum_case_buildup/viewall.html  

Thanks for the link! Tranny is coming out in January, going to go through the entire unit. I'm thinking of just replacing the entire unit with a new one as the rest of the car has been totally rerstoered. I'm assuming the trans had no issues at the point of the resto as it was the only component that didn't get restored. Definetly want to keep it with the 4 speed O|D unit. The launch in first to second is incredible! Just wish it had a bigger fuel tank!lol  
76 Feather Duster
360ci, 4speed manual(overdrive unit)
355 posi 8 3/34 rear end
edelbrock high rise aluminum intake 650 cfm carb
Msd 6al digital rev limiter ignition
Flo-tech headers
2 1/2" dual flow master exhaust

do you still have the factory Thermoquad carb???   was it restored or rebuilt???



fuel economy issues on a 76 can be caused by a lot of things...

including driver inputs...

vacuum leaks
EGR valve leakage and proper operation
carb adjustments  (this takes hours to just discuss. )
ignition timing curve incorrect for the application
slack timing chain.
improper PCV flow..
dragging brakes
improper wheel alignment.. usually incorrect toe in..
clogged exhaust..

you might want to see about having an oxygen sensor bung installed in your exhaust pipe . and install an oxygen sensor then an air fuel ratio display...  the display can be installed as a temporary unit... magnets to hold it to the dash board...  power through the cigar lighter cord.. so it just plugs in and a single connection to the sensor..

there are a lot of options from cheep to NOT CHEEP..



for dual sensors


all the way up to the expensive ones..



these allow you to monitor your exhaust output... so you know where in the fuel system to change the settings or jettings for most efficient fuel economy..

you might want to make the magnet mount gauge bracket a double and include a vacuum gauge as the second gauge.  

Well had the old tranny rebuilt front to back. What a mess she was inside!!! 2nd and overdrive were chipped pretty bad. Front and rear main bearings were showing excessive play as well. New Unit in and she works like a charm! As for the carb question, it has an edelbrock 650 cfm. Works just fine, but the car was originally a slant six and has a very small fuel tank lol. Gets respectable fuel mileage actually now with new transmission and wheel bearings all around(they all were worn out). Dumped a pile of new parts into her this winter and all i got left to do is some interior upgrading over the next while. Time and money, have plenty of one!!!  
76 Feather Duster
360ci, 4speed manual(overdrive unit)
355 posi 8 3/34 rear end
edelbrock high rise aluminum intake 650 cfm carb
Msd 6al digital rev limiter ignition
Flo-tech headers
2 1/2" dual flow master exhaust