The controller vibrates when I'm near them, but I don't know what to do.
Screenshot of one of them on the map:

The left and right motors in your controller will play a pattern when approaching these. It will pause briefly before repeating. You need to mime the pattern back with your left and right triggers.
My vibration was off, so I missed this the first time through. If you're having trouble, the right motor was a higher pitch in my case. Instead of feeling the sides, I listened for the pitch.
As far as I know, across ports, you can't solve this on your own without a vibrating controller.

Here are the codes, if you're lazy or without:
LT RT RT RT LT RT RT LTLT RT RT LT LT LT RT RT RT RTLT LT LT RT LT RT LT RTLT LT RT LT LT RT RT RTIf you're playing on the PC, while you're standing near the tuning fork a low tune will be will be played on the left or right earphone. Press the corresponding key (A or D) to rotate the perspective following the tune.
I checked with my headphones it's very subtle but it's there. So, I think with normal speakers you'll definitely not hear a thing ;-)
LTand theRTrumble sounds. – Rory O'Kane Sep 22 '13 at 23:50