i sometimes have a problem with dropping into reverse and just letting it sit there for a sec and it will make a groaning noise for about a couple of seconds. i changed the transmisssion fluid and filter and transmisssion seemed to be running good.

 

Is this when you're first starting the truck and backing out of somewhere? If so try putting it in neutral first for a minute and then reverse. Does it still groan?

 

my 98 ram 1500 makes a groaning type noise in reverse until the fluid warms up. i found tsb 21-08-99. it explains that it could be the transmission regulator valve

 

its a common problem, but not serious. ignore it and drive on.

 

What he said

And the TFOD JR shift kit can fix this and a few other minor 46RE problems.