A few technical questions about TC4

[emvali]

I am a little concerned with 2 issues I have with my TC4 which I recently purchased.

The machine was mainly used on plastics and aluminum by the previous owner and this is my regular use too.

The first thing, which I simply don't know if it is normal with these machines or not, is the spindle's high pitch noise. The spindle type is 80-8000rpm no gear.
Up to 3000rpm the machine is very quiet, raising to 4000 starts making a high pitch which at speeds of above 5000rpm is disturbing. I tested the machine at 6000 and up to 8000 - the high pitch is causing me to look for the ear caps - it is really loud.

My question is, this is the normal noise or there is a problem?

The second issue, which is for sure a mechanical problem is a knocking noise I get from the X or Y axis (or both).
This knocking apparently happens with certain tool paths:
With circular or rctangular motion everything is ok.
I made a part with an octagonal shape pocket, when reaching the 45 deg angles there is a clear knock I hear under the table. I am almost sure it sounds like something moving in one direction and knocking and then back.
To explain myself, if I go on the octagonal shape counterclock, say the mill goes on the right side up (Y) then 45 deg to the left - here makes the first knock, then goes XY on the 45deg and again 45 deg to continue with X only, there makes the second knock that sounds like something is back to place after the first one.

On the 8 angles of the octagon, the knock is in "pairs" at the bginning and end of the geometry segments involving XY axis moving together, each time where the toolpath switches from X to XY and then to Y.

An important thing is the knocking happens also with no load at all.

The knocking is not affecting the surface finish or tolerances, but is making me worry.

Any ideas where to look for the problem?

Thanks