Unevenly worn tyres or tyres on wheels at different stages of wear will do that. Sometimes it will manifest itself as nosewheel shimmy. Also if the aircraft has been standing in the cold for a while the tyres will have flat spots that take quite a long taxy to even out, so if it is just a short taxy before commencing the take-off the vibration will be quite noticeable. Particularly so with low pressure tyres.
The cause of the flap 'howl' has been covered elsewhere. Search and ye shall find.