Motorboat sound in pipes

[bwhite]

For the last couple of months the cold water pipes to my upstairs bathroom have been making a load motorboat sound every morning...this happens several times and then stops for the rest of the day. It seems to start when my tenant upstairs uses one of the fixtures (toilet/shower or sink). I can feel the vibration in the cold water pipe leading to the upstairs area (but it's definitely not the pipes "banging" together). The cold water supply for this area, taps off of the water heater inlet line. It's driving me crazy...any help is much appreciated. :confused:

[plumber1]

My best guess is the ballcock in that upstairs toilet.
Could also be the toilet stop.
The washer could have broken or come loose
While your upstairs, go to each cold water tap and very slowly open them one a time to rule out a bib washer or cartrige.
For six or eight dollars, replace that ballcock.