Focus Mk 11, 1.6 hdci, 90 hp, estate thrumming noise.

Hi chaps. To cut a long story not so long. Car went onto limp home mode and the warning cold start fault came up on the diagnostics. Checked the glow plugs, all were shot. When removing the last one, I Broke the tip off and it fell onto the piston crown😱. Started to remove the All the things that needs removed when pulling the head off, and then read about some guy fishing a broken end bit out through the injector hole. Anyway, it also worked for me🎣 the thing is, since I’ve put all the bits and bobs back together I get a thrumming noise when I first start the motor between 1500 and 2200 RPM. But once the motor nearly reaches operating temp, the noise is almost gone. It’s fairly driving us nuts. I’ve been around about under the engine, checking various things for tightness etc etc etc. Anybody had this before and did they find a cure?