Tranny noise and sliping - 2004 Buick Regal

your guessing right too, its a drive belt sliping check all the belts as the one thats sliping will soon brake

could be throw out bearing any vibration?

tranmission is sliping or low on fluid.is there a lot of miles on truck.engine not lossing power transmission is sliping.

RX350's are are known for blowing trannys.(I work in a Toyota dealer) Id try a nice tranny fluid flush to try and save it with nice new clean fresh fluid. Also check for stored codes in ECU or tranny controle module to make sure it isnt just a bad shift solonoid in the valve body. If the flush dosnt work and it still does it id look into bringing it to a tranny shop and see if it can be rebuilt or it may need to be replaced with a new or used one.... I have seen alot of tranny jobs been done on those vehicles before.

check tranny cooler core and finns may be restricted

Tranny is gone. Is it a manual or automatic?

sounds like your tranny is going to hatch at any time. each mile that drive could be your last. i would get this thing off the road and get a new tranny into it.

it most likely is watch your rpm's use your bets judgment if you step on it and your rpm's jump and it is slugish then its slippin.

I am willing to bet that the clutch packs went out since it was that low on oil because there are 2 pints in a quart and that is a lot of tranny fluid to be down i am going to say you need a rebuild on the tranny

may be the filter . this is what filters the tranny fluid . and if clogged or dirty will prevent fluid travel thus giving you slippage . can be bought at auto part store and put on your self in under an hour. if it started sliping and slowly got worse then it very well could be. now if you dont stop driving it you can blow it up.