Hello everyone.

I purchased a Chinese spindle via AliExpress. It is a 1.5 kW, 24000 rpm, ER16 liquid-cooled spindle.
When I run the spindle above 11,000 rpm it makes some kind of rattling noise like something is interfering inside, but the shaft spins smoothly and below 11,000 rpm the noise disappears. There is no resistance felt when I spin the shaft with my hand. It is perfectly smooth. I contacted the seller and he said it is normal and that I continue to use it. But the noise is quite worrying me.

https://www.youtube.com/watch?v=LrzEDfsBpPY

Since I was just testing the spindle for a few seconds I did not connect the water cooling tubes yet (in the video). Later I did a long run (running continuously for about an hour) with the tubes connected and water flowing through it, the spindle seemed to be working fine, no overheating or reduction in speed except for the noise it keeps making.

Can anyone tell me what is the problem? Is it all right to continue using it? Or what can be done to stop the noise?

Thank you !