Cetus3D printer knocking during fast movements

When the print head moves fast, X or Y, there is a metallic sounding knocking noise.  At first I though the plate was flopping around or hitting an object but it's not.  The print seems okay.  The fast movements are when it's moving to a new XY location during the print job, not when it's laying down material.  Anyone thoughts?  I'm on my 5th print job, so far it's been performing well.  Thanks.

I have this occasionally.  My pulleys are very slightly out of alignment, causing the belts to wander off-centre, and I think the noise is the belts thumping against their channels in the extrusions.

I've had this happen and have traced it down to the extruder slipping.  It seems to happen when the print head is moving to a new location or changing direction and tries to withdraw/retract the filament slightly.  Setting the print speed to "Fine" fixed things for me most of the time. Happened more with the 0.2mm nozzle than the 0.4mm. 

We are absolutely loving our Cetus3D printer. We just ran out of the spool provided with the printer and began using eSun PLA+ filament and we noticed the same knocking sound coming from the print head. I followed the advice from kingofahyon above and the knocking has stopped. I have the Temp set to 215 as well using the custom setting.