CH radiator pipe may be leaking under concrete floor?

When the CH system starts and for approx 10mins thereafter, I can hear what sounds like a slow dripping noise underneath the floor approx 3ft from the nearest radiator. The same noise occurs when the system switches off, but for longer. This sound can't be heard once the system has been active for a period of time so I'm uncertain whether this is actually a leak or pipework expanding/contracting, etc. I'm not able to check visually as the pipework is underneath a concrete floor. Has anyone experienced anything similar and can offer advice?

Angela
November 2013