Usually that noise is trapped air around the valve ports that isn't being flushed out. Pressing down on the handle while the pump is running can damage the spider gasket. If it isn't glued in the channels one or more of the spokes can lift out and when the handle is released it will be crushed and deformed. This explains the water out the waste line. Usually that chatter noise will go away in a little while. Mine does it periodically but shuts up after 15 minutes or so depending on how much air I got in the assembly. If the pump isn't ramming enough water through the thing the air may never be purged. Hope this gives you an idea what to look for.
Al
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