Had used transmission installed 5 days ago - just started making loud chattering noise - what could it be?

i own a repair shop,and it sounds like he didn't tighten up the torque converter bolts real good on it,they may actually be backing off in side this thing,and if so it may do some damage to it but that's what it sounds like to me that's happening to it,id take it and have him check it again,good luck i hope this helps.

If it was only 5 days ago first thing I would check is if anything is leaking, like automatic tranny fluid, also check the fluid level and the color, it should be a nice red color on the dipstick. I am thinking by "shadetree" you mean not replicable just a backyard mechanic that doesnt have his own shop in which case if he doesnt want to help you out legally there is nothing you can do unless he signed some kind of warranty for his work.

So many differant things, could be the fly wheel. Try to find a transmission shop that will take payments and don't use shade trees.

call the mechanic stop driving. Some good advice Pay Peanuts Get Monkeys,

Did he warranty the trany? Most of the time a used car part that has wear will fail quite quickly.

take it back to guy that did the work and either get money back or have him re-do it for free...