If you put in 0/30 rather than 10/40 then you could well have metallic noises at lower rev's. This is due to lower oil pressure.

Things like hydraulic tappets could produce the noises you describe.

Engine power would be uneffected, however the thinner oil can give less protection than the correct oil for the engine, and could cause failure of main bearings etc.

As the rev's increase the oil pressure will build and the noises would stop.

Do you still have the can ? if so check the grade.

It's easy to pick up the wrong grade, I did it once, and after a few day's my oil pressure light came on at tickover.
Realised mistake, put correct oil in and got away with it. If anything it acted like a good oil flush.