OR you can just not cut it at all, and use the long pipes with epanded rib to help waterproof the connection.
I have 2 pipes coming from my heatercore. I noticed that when placed in my case the 2 pipes rise above my 3 1/2 bay without obstruction. I cut a rectangle hole for the tubes to rise up and through my empty 5 1/4 bay. The tubes going up free much of the airflow around the radiator area for air, and keep the case from looking cluttered.
To connect tubing to hcore, go to NAPA or some auto parts store and get a tube adapter that fits in the larger than needed tubes, and will fit into the 3/8 ID tubing. Cover the adapter and tube connection with hose and tighten with 2 clamps. If you use an Antec case like mine, the 3 1/2 bay will cover the hoses and adapters leaving a nice looking clean case. Mine is not the cleanest now, but give me time
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I have 2 fans tiewrapped on either side of the core in push pull config. (yes I know I dont have a shroud) Suggestions welcome.
back on topic, I think the rattling is loose soldier and will not effect performance. Cover one end with your finger, submerge in water in sink, blow in other end. If you dont have bubbles you should be ok.