worcester 241i RSF thumping noise?

My bolier now seems to making a thumping noise when I turn the heating on (not when the hot water's on) thought it may have been air in the system so bleed all the rads & topped the water up as I noticed it was slightly lower than norm...alas to no avail, the heating is working but still makes the noise every now & again. Have just been made redundant so can't afford large plumbers bills & would really appreciate any advice.

cfwar25
December 2008