I like what mountainbike said
In my professional experience, sway bar links and bushings have caused some surprising and horrendous noises. Meaning the noises are horrendous, and you’re surprised how quiet the car is after making the repair
@tsm: Hmmm, worn bushings…just had those replaced on the rear; perhaps the front bushings also need replacing. That is where the thunk sound is located.
Also, just got back from a very nice long drive for over an hour in the countryside. Left the radio off and paid close attention to the sounds and feel of the car. Transmission behaved perfectly with no slippage, which was nice. Thunk was noticeable and happened about 20% of the time the transmission shifted gears.
Checked the transmission fluid when I finished the drive, going through the proper procedure of putting on the parking brake, placing the car in park, then running the shift lever down through all gears and back up to park, pausing about 3 seconds in each gear, then pulling the stick, wiping it off, putting the stick back in, leaving it about 5 seconds, then pulling it again to read the fluid level. The fluid was right on the mark and a clear, clean red.
Running through the check procedure I noticed that the thunk occurs the most and loudest when shifting into or out of overdrive and third gears; info which I shall document for the mechanic.
Since the bases have been covered I’ll just revert back to a prior suggestion I made about a road trip on another one of your car problems… 
LOL Too funny OK, too funny.
Impala is front wheel drive? news to me. Oh well,
https://www.google.com/search?q=fifteen+days+under+the+hood&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&channel=sb
Really? There hasn’t been a rear-drive Impala in 20 years!
Wow,I guess my service life should be rescinded, I suppose the next thing would be points are obsolete!
UPDATE; Transmission hasn’t slipped anymore, so apparently it was simply my brief sloppy lapse of proper shifting habits.
However, had discussions with both shops, dealer and independent, who have done all the work on the car since I bought it, regarding the full history of everything that has gone wrong at what age/mileage and the current developing problems and got quotes for repair costs. It adds up to 50% to 75 % of the car’s current blue book value. And that doesn’t add in the cost of doing normal maintenance work such as brakes and muffler which are close to needing replacement.
Indy shop pointed out that the warranty repairs done at the dealer replacing trans cooler lines, intermediate steering shaft, power steering pump and rack, and water pump would likely have used parts of same manufacture as the originals that failed so early and therefore might be prone to early failure again. The dealer shop was more discreet but as much as admitted they have been seeing the same.
I asked the indy shop, bottom line, if I were their mother, wife or sister, would they keep me in this car. Answer, probably not.
So, I have spent the past couple weeks doing a lot of test driving and research, complete with the CR buying guide and lots of internet sources. So far I haven’t found anything in my price range that I am comfortable with. But, I’m not done looking. Worst case, I give up, throw repair money at this one for now, drive it another year or two and hope. More likely I’ll make some compromises, choose a new car, and live with the choice. At least it will be under warranty for a few years.
I do have a firm price offer to sell the car at considerably over current book value due to the extremely fine condition of the body and interior of the car, so that helps. I’m making the purchase transaction separate from any trade in deal. Get the best price on the new car. Then see what is offered on trade in of this car. If less than the offer I have in writing, I’ll let them match it or I’ll just take the car to the used car buyer and sell it for the amount offered.
Never thought I’d replace a car at such low mileage and few years but it’s looking like that might be the wiser choice. I really don’t know and am frustrated by indecision. But the fact the indy shop which gains nothing by suggesting it is time to get a different car has stated they believe it would cost more over the next five years to keep this one versus what I’d be out of pocket for a new car, has me leaning toward new car. Now if I can only find one as comfortable and quiet as this one!