Car Makes Thump noise when in reverse?

So its been happening for a few months now. In the monring when I shift the car into reverse, it tends to make a noticeable thump noise or bang noise then a delay and finally jolts into reserve. My friend said its just because the cars cold and old but want to get a second opinion. The car is a Honda GSI 1994 Auto. When it starts up the revs are pretty high, it stays around 2,000rm for 5 mins then it drops down 1k and I normally don't get that thump jolt crap. Just wondering what i can do to fix, prevent or treat this.

One problem at a time:
Pull your trouble codes, even if the check engine light isn't on you should always pull your codes before trying to repair a problem, then fix the issues that the codes present as this may solve the issue(s) that you are having. If you don't know how to pull your codes you can follow this video, it's for a Civic but same concept.

Reverse. You need to let the car warm up for about 5 minutes, preferably taking it on a short drive instead of just idling. Park the car on flat ground and check the automatic transmission fluid. If it's low, which I'm guessing it is, you need to fill it to the proper level, only adding about 1/2 qrt at a time. If the level is within spec then it could be one of your shift solenoids is bad, could also be time to change the transmission filter. Both of those should be done by someone that knows what they are doing, neither one are hard and could be done in the driveway if you do enough research beforehand.

High RPMs
Could be a number of different things, will usually be a code thrown which will narrow the options down. IAC valve and throttle body should be cleaned using throttle body cleaner ($3 @ walmart), and a toothbrush and rags. You don't have to get everything on the TB but the important parts are the back of the throttle valve and the sides of the valve/TB where they meet, it should be perfectly clean to allow it to fully close at idle. When cleaning the IAC don't let the TB fluid pool or build up inside the component, keep the plunger facing down and spray it down, wipe it off, you may need a rag to get the nooks and crannies. Just don't force the plunger to move by hand (this part may need replaced especially if it's original equipment). High or erratic idle can also be caused by a vacuum leak in one of the vacuum lines or fittings coming from the intake, you will need a vacuum gauge to test for sure, pull each line one at a time from the intake and apply vacuum, if it holds then reattach and move to the next, if it doesn't hold vacuum then you need to isolate the bad part of that line. Each fitting should be tested as well as each section of the line, otherwise you can just replace the entire hose.

If you haven't fixed the idle issue after these steps the you will need to connect the vacuum gauge to the brake booster line (biggest hose coming out of intake going to a circular object usually mounted to the firewall), make sure one end of the line is still connected to the intake, the other to the gauge, start the car, you should hold steady at 17-23hg, anything else and you have issues. If you watch this video it explains all of the things you can learn from engine vacuum.