I wasn't sure how to post a new thread (duh) but wanted to share with anyone having the same problem as us.
Today we had our 4th engineer visit (our Worcester boiler is insured) to sort out an intermittent droning noise coming from our boiler. We have had parts changed, we've bled radiators, turned rads up/down, pressure dials down, had system drained, you name it but the problem remained.
Today, the guy said instantly that he knew what it was. He changed the internal settings, to do with the 'low burner' and it stopped. To prove his point he put the settings back to how they were when he arrived and it started again.
The settings are inside the boiler so us lay people wouldn't know they exist, apparently the problem is to do with the settings when the leaves the manufacturer. Our guy rang Worcester and double-checked what the settings should be. So, for those who are experiencing the problem or are asked to investigate, I hope this helps