Vibration like feeling while pedaling

[Kajjal]

When I have had this the causes were :-

pedal bearings needed tightening and regreasing
front headset bearing gone, replaced bearing
loose Chainring bolts
front mech cable end rubbing on tyre
rear wheel bearing gone, ending up replacing both wheels good excuse ( take wheel out of frame and spin it was easy to spot.)
rear wheel bearing loose and needed regressing.

Generally speaking the more power / effort put in to pedalling such as up a steep hill or sprinting the worse it was.

 

[Brooks]

I'm more than sure it's the Arsenal bracket!

 

[gbb]

Has the previous owner or supplier fitted a new chain ?
Quite a few times in the past, when ive fitted a new chain to an existing chainwheels and cassette setup...i got a mechanical whirrr.
Iif thats whats happened in your case, it will pass with a few rides, i think its the slight mis match between new chain and worn sprockets....its just bedding in.

 

[silva]

There is a tensioning up and down, which indicates an out of center and/or a concentrated wear somewhere, an idea could be "repositioning" the chain on the mostly used ring, ie rotate a crank to an easy to identify pedal position, mark the link on top of the mostly used ring, then losen the chain, then select a chain link (ring teeth number divided by 4) away, and place that link on the top position.
Another option is to rotate the position of the chainring(s) on its mount (spider).
Check the chainring teeth for sharkfin shaped series. Those make it harder for links to disengage, and aggravate this wear.