Are "thank you" or similar notes allowed in questions and answers?
I believe that they are frowned upon on some other engineering (stackexchange) forums.
Saying things like "Hi!" or "thanks in advance for answering" are frowned-upon, partly because they just clutter up the question. Also it's a bit cheeky in a question to thank people for an answer, it sounds like you are putting pressure on them to answer.
You will probably find some higher-reputation users removing such notes from questions, although I think if you are going to edit a question, you should be making more improvements than just removing "Thanks" from the end of it.
As Jeff Atwood noted there are better ways of thanking people for their answer like:
I'm not sure why you would put "thanks" in an answer. Who are you thanking? However again, it just adds to the clutter to do it.
You are not supposed to make a comment to simply say "Thanks!". A more appropriate response is one of the ones above. The person answering will notice the reputation boost from voting up their answer, and certainly if you accept the answer it is clear who did the accepting.
However if you have something useful to say in a comment, then adding in a "thank you" won't do any harm. For example:
Thanks for that great answer. I found that if I increased the sampling rate it worked even better.
In that case you are contributing to the answer with your technical note, and slipping in a "thank you" at the same time.