I would say "There is", but that's OK.
You have to turn the sentence around to see if it makes sense and is singular or plural. In this case it would be as follows: “A lot of rumors are flying there on this thread”, so I would say it is correct to say “There are a lot of rumors flying on this thread”. It would not sound right to say “A lot of rumors is flying there on this thread”. In other words, take the sentence apart and just use the subject and verb and you have “rumors are flying”, not “rumors is flying”.
Have I made myself clear? There will be a quiz on this.