Unwanted noise in oscilloscope display
Looks like it's EMI noise coupling into your measurement from your power supply. Make sure that the power cord for the scope is as far away from your probe as possible. 

To confirm it is noise from your power supply check the frequency by measuring the peak to peak time difference and calculating 1/(t1-t2).

Another hug help could be reducing the ground loop of your probe by using the tiny little ground ring at the end of your probe. You could wrap a peice of wire around it or a paperclip. 

You ground loop has a current generated in it proportional to the change in magnetic field flowing through it. Smaller loop means less change. 

Good luck!!

Also, and amplifier circuit could help scale your piezo output from say 0-0.5V to around 0-5V, while even filtering some noise out for you. Google low-pas filter. Cheers!