why is there a ticking sound on my p990i when i attach my headset?

why is there a ticking sound on my p990i when i attach my headset?
I have a SE P990i and when i connect my headset i hear a ticking sound when i'm speaking to someone. If I use it as listening to music there is none. I am not sure where could be the problem. My headset or my phone. Thank you.