New 50 Amp Plug

Check it with your voltmeter.

Put a voltage meter on the outlet and measure it for yourself. Across the 2 flat blades on the sides you should have 240VAC. From each flat blade on the side to the flat blade on the bottom (or top in your case...LOL!) there should be 120VAC. And likewise from each flat blade on each side to the ground, there should be 120VAC.

Then, test the outlet with a voltmeter!

Both outer straights are HOTS

Center straight is NEUTRAL

Center round is GROUND

So, from either outer straight to center round or center straight is 120 VAC

Center round to center straight is 0 VAC

Outer straight to outer straight is 240 VAC

I assume it was the socket that was installed, not the plug.

I don't think there's an official "right side up" or "upside down" for the NEMA 50A socket. What's done for RV's may be a loose convention, but is not code. The same socket is used for electric ranges and in that use often may get installed in any which-way due to space constraints and baseboards and so forth.

The hum is likely just the coil for the contactor inside the surge guard, and if so is not abnormal (and could have gotten louder if some mechanical part got ever so slightly looser over time). If you want to make sure the socket is wired properly, you can always verify the voltages manually with a meter.

I can understand your concerns....

With the 50A Receptacle 'upside down' ie: GROUND at bottom vs at top the Surge Guard plug is upside down so it doesn't hang straight down

50A is hard to screw up voltage wise ---unlike 30A which can be 120V or 240V
50A ...2 hots, 1 Neutral, 1 Ground
SOoooooo
Turn the two 50A circuit breakers OFF
Pull Receptacle and reorient it so the GROUND is at top (then surge guard can plug in and hang straight down)
To reorient....remove wires/rotate receptacle/connect wires
Hot on each side (Black/Red Wires)---Neutral on bottom (White Wire)---Ground on top (Green/Bare Wire)


Then plug in Surge Guard then after it completes testing and allows AC Power use it

your Surge Guard is aging.. What is causing the "HUM".

The solenoids consist of a wire coal around a laminated iron core.. This core is made up of many "Leaves" of iron which are varnished and stuck together before the varnish dries.. When current flows in the coil a magnetic field is generated which causes an armature (moving part) to "Pull in" and contact (on the end of the armature) is made.

I won't talk about the electroincs only the solenoid and most specifically the iron core.

Now the coil is AC

As the unit ages the varnish that holds the leaves starts to dry even nore and eventually it starts to flake off. This allows the leaves to vibrate and :"HUNM"

Another source of HUM is if something prevents the armature fromn pulling ALL the way in (Contact is made at around 90% pull) For example an insect on the core end.. then the armature can "hum" but I've not seen this save on a switch where the insect got not on the armature, but the contact, and I had to go down and clean the thing so the pump would work (My parent's water pump)

I installed the 30 amp outlet behind my garage "upside down" as there was a plastic cover that needed the wire to hang down and it couldn't go in the way I first installed the outlet. spose a 50 amper would be similar if it had a hinged cover.???
bumpy