Vibration from pedals at slightly higher speeds?
You may be feeling vibration coming from worn bearings in either the pedals or bottom bracket. Something this is only discernable when applying greater force to the pedal and crank - which would explain why you feel it in the down stroke but not return stroke.
Manually turn the pedals feeling for excessive resistance, 'notchiness', clicks or play. Do the same for the cranks. If you can slip the chain out of the front derailleur cage and off the chainrings it makes it much easier to feel any problems when turning the crank, because there is no drag or vibration from the chain.