I am going to drill a 1.5" bulkhead no matter what. My question is where to place the pump, which will change the plumbing for the inlet and outlet. Existing bulkhead location, straight into the inlet and then needing a 90 to get the outlet to the back of the tank, or Existing bulkhead location, run to back corner of stand with PVC including a 90 and then a 45 to get to the back of the tank, or new bulkhead in back corner of sump, straight to the pump inlet with 45 to get to the back of tank for return.
Just a note: I pulled my last post, prior to the reply, and there was a reason for that. Unfortunately, personal attacks are a poor tactic, under any circumstances, so it is hardly worth the time to pursue, or otherwise comment on that topic.
On with your issue.
If the noise was not a 60Hz hum, and it was not the fan, (unsafe to assume) however it is probably one of three things: The pump was cavitating, the motor is damaged due to cavitation, or the motor is getting ready to quit. (dry bearings what have you.) It has been a very long time since I have run these small pumps, but I do have a 30 year old little giant that is silent save for the fan and 60Hz hum.
The reeflos and wavelines are the quietest pumps I have yet come across.
The Wavline pumps, come in two sizes (ignoring flow rates): ~ 7" x 4" x 5.7" (DC 10000, DC 12000) and 6" x 3.6" x 5" (DC 4000 and DC 6000.) The inlet size on the smaller pumps is 1.25", and 1.5" on the larger pumps. Outlets are 1" and 1.25" respectively. So they are smaller, in some ways. No way the reeflo pumps are going to fit in the stand, and a 1.5" bulkhead is what you want to use, unless going with larger inlet sizes (1.5",) and moving the pump out. See below.
Note: the waveline pumps are submersible by default. You need to obtain a volute gasket to run the pump external. Unfortunately, you decided to make the return section too small for the pump you are already using, so there is no room for a submersible pump, even though that would be the cleanest simplest solution to your issue.
Assuming the pump will be external, if you cannot have a straight run to the volute > 10" from the last elbow in the intake plumbing, don't put the pump in the stand, rather go to plan A which was to move the pump, with the necessary plumbing modifications. With the intake plumbing either 1.5" or 2" depending on the particular pump you select.
At any rate, don't do anything to the sump, until you have finalized everything else.