Does the pressure in the filter go up when in spa mode?
More than likely the spa has more return head loss than the pool. This tends to quite pumps some. Lower flow rates means less turbulence in the pipes. Also, higher return loss reduces suction which in turn reduces impeller noise.
If I pump from spa to spa... very quite, from pool to pool is noisy, if I pump from pool to spa sounds ok, but circulation in the pool is poor and less debris makes it to the skimmer.
The 2x change in filter pressure is an indication that the spa has a lot more return loss than the pool. So I suspect that the pool is running at very high flow rates. Do you see any air in the pump basket when in pool mode?
This is a pretty common situation and can be solved in a couple of ways. You can add some head loss to the pool return but that would decrease flow rates, increase turnover time and reduce the pump efficiency. If you are ok with that, there are several ways to do this.
1 - Replace the return eyeballs with smaller openings.
2 - If you have a return valve which can control flow, you can restrict the return flow slightly.
Otherwise, you might try to sheild the pump a bit better although some of the noise is probably coming from the plumbing as well.
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