Problem with New Bike

My neighbour had a bike with alu frame and 27in wheels which he used for
long charity rides until it got nicked.
Now with his insurance payout he has a brand new bike with steel frame
and 25in wheels professionally set up and he is getting some training
time in over the holiday. He complains he can feel a difference in the
new bike when he's standing on the pedals and he says its like a
'resonance' but he can't say where its coming from.
Its to go back to the shop for its check but what do you think could be
causing it?