Just for an idea (I have never owned an Eheim)
When my aquarium pumps do this and I have had aquarium pumps for... 10+ years now... it usually means the impeller is dirty and is causing the rotation to be thrown off. Therefore the sides of the impeller are striking the walls. Usually what I do to fix it, is completely remove the impeller and clean it and all the interior walls with a pipe cleaner.
The problem here, is the pump is starting out like this. I don't like it. I may have taken it back and exchanged it as another user may have not cared. The only positive note, once the bacteria and goo starts building up it may coat the outside of the moving parts and buffer the sound and workings. AND eventually that impeller may strike the surface and wear down and balance itself. (don't hold your breath)
While putting on the new fitting, do you think you may have bent part of the casing closer to the impeller?
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