I believe the grinding noise is normal. I bought mine new and it's been making a noise that I'd describe as grinding ever since. The unit works fine, however. RCMs are pretty crude in design and I think motor noise reduction is not really a top design priority given their purpose.
I too had the same problem as the OP at the beginning, initially solved by using a rubber washer under the plastic knob as otherwise I had to tighten the knob to the point I was concerned I might either break the knob or damage the record label. For some strange reason, the problem worked itself out and I now don't use the washer and don't have to tighten the knob as hard for the platter to keep spinning with the vacuum pump on. Seems the parts needed simply to break in.
I too had the same problem as the OP at the beginning, initially solved by using a rubber washer under the plastic knob as otherwise I had to tighten the knob to the point I was concerned I might either break the knob or damage the record label. For some strange reason, the problem worked itself out and I now don't use the washer and don't have to tighten the knob as hard for the platter to keep spinning with the vacuum pump on. Seems the parts needed simply to break in.