I have a squeak that seems to occur on every third rotation only when I'm applying real pressure such as climbing or accelerating hard. Seems to be from the rear but hard to tell. Had the bike (2014 Tarmac w/ultegra)serviced and it was quiet for about 100 miles and now the squeak has returned. Any ideas? Thanks!
Third rotation of the cranks or the wheels?
There are many potential sources of noises on the bicycle. Along the way, you have to determine if the noise is related to crank rotation, wheel rotation or neither.
Crank rotation possibilities include, but are not limited to: seat post, rear q/r skewer not tight enough, dropouts, frame, chain, ...
Wheel rotation possibilities include, but are not limited to: spoke tension, tire defects, rim defects, freehub, cassette, .....
The chain takes more than one rotation to complete a circuit of the drive chain, so a gear combo such as 42-14 would bring the chain around once every 3 crank revs (42/14=3). The chain usually makes the most noise as it passes through the pulleys.
Also when trying to track down a creak, squeak or whatever the sound, it is important to try to pinpoint, that you try one thing at a time, then test ride and go from there. If you do 3 things at once and the squeak is gone, you do not pinpoint what the actual cause was.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
Could be a bent rear axle spindle that is what happened to me with the same thing.
Every third rotation of cranks might point to a chain issue. It typically takes 2-3 rotations of the cranks to make the chain run through it's whole length one time. Most other parts would squeak in time with the crank or wheel rotation. But squeaks are notoriously hard to track down so YMMV.