Sounds like it’s pulling into a vacuum on inlet and high pressure outlet, which explains the roughing.
But It does not sound 100% though. A perfect condition cmep anything is a strong hum, rather then an obnoxious knocking engine sound. Get it rebuilt before it creates expensive crankshaft problems.
P.S: I wouldn’t even pull past 0psi with a cmep. Doing that exposes its design flaws. I learned that the hard way. 
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That’s the bearings. Time for a full rebuild. Definitely heard that sound before.
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Man I fucking hated that pump. Just buy 2 of the smaller ones and run em in parallel
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But ours used to sound like that a lot. Our mechanic never fixed it. I’m gonna rebuild ours cause every thing she ever fixed always ended up way more broken after the fix… lil guys work well though. And at a fraction of the price you can get a few and have a backup
Could also be nitrogen thats being pumped through the cmep. I had similar sounds because of that
Yes, it’s normal for a chinese made piece of garbage to sound like trash.
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welp i just got it rebuilt not even a month ago…
Kinda sound like its made due to high pressure.