If there is a flow restriction and the pump wants to spin faster, vapor bubbles can form in the pump impeller. This is known as cavitation ...
You will likely find nothing in the pump or around the impeller unless the pump is running.
Turning the pump speed down a notch shows the pump has exited a cavitation problem. Returning the pump back to the higher speed only brings the cavitation back.
If something was in the pump, you would notice it at all speeds.
The Laing pumps are a Maglev type.
If the pump was ever run dry or the cavitation became so bad that a squealing sound starts; disassembly the pump, dry things out inside, put 1-2 drops of 3in1 oil on the white ceramic ball, place the impeller back on and by hand rotate the impeller back and forth. This will allow the plastic in the impeller to become lubricated again. 3in1 oil is compatiable with our loops of copper, steel, and plastic.
Do not use "grease" type lubricatants in the pump. It will make a mess of the loop and sludge things up.
Sounds like cavitation happening with the pump. That is, the loop is too restrictive for the pump to properly draw enough fluid and vapor bubbles actually form. Having the pump run faster can cause cavitation if there isn't enough flow to supply the pump.http://en.wikipedia.org/wiki/Cavitation
Image above shows an impeller much like the MCP350/355 D4/D5 MCP655 pumps ...
Images below are like the Ehiem and other types.
Suction Cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition has large chunks of material removed from its face causing premature failure of the pump.
Discharge Cavitation occurs when the pump discharge is extremely high. It normally occurs in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater similar to what occurs in a venturi and turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition due to the high pressure condition premature failure of the pump mechanical seal and bearings can be expected and under extreme conditions will break the impeller shaft.
Try to reroute the loop or remove the loop restrictions.