Is the Lindell on a powerstrip ? can you try to put it directly to the wall and tests also with a ground lift ?
If you put back the Phoenix alone in the same slot it was, is the problem still there ? If not try the P1 alone, still there ?

This is likely to be a fault that has developed in the power supply of the 500-series rack, maybe brought on by it being asked to power two modules now rather than just the one you had previously. You don't say this, but I assuming the amplitude of the hum is dependent on the gain settings of the module.

I have had an expanded look at the clip you posted and it shows a 60Hz (rather than 120Hz) fairly clean sinusoid. On those Lindell 503 units, the +/-16V rails are delivered through a switch-mode power supply, but I don't know the details of how the 48V line is generated, and I can't immediately find any schematics on the web.

I have seen this type of effect once before, many years ago. If my memory serves me correctly, it was one of the smoothing capacitors on the 48V phantom power line which had become leaky at higher voltages. Taking some power from it by plugging in a phantom-powered mic dropped the voltage enough for the leakage current not to be a problem. In your case, however, I'm surprised that the waveform is at 60Hz and near-sinusoidal, as these leakage problems almost always generate harmonic currents. In addition, mains-frequency problems are much rarer in a chassis that has a switch-mode power supply.

You could provide us with a little more information:

(a) does the amplutide of the hum level vary with the gain of the module?
(b) when you say the hum is worse with no phantom power switched on, does that mean that a dynamic mic (PP off) works correctly but with added hum?
(c) if you have one module in slot 1 and the other in slot 3, is the hum level on the slot 3 output a function of whether there is a phantom-powed mic connected to the module in slot 1?
(d) when you talked about the hum level with no microphone connected, was the input XLR open-circuit? If so, does it make a difference to connect it to the output of a quiescent separate piece of gear so that it is seeing a low output impedance?

Thanks guys -

I have tried single units - and yes, just a single module in the 2nd or 3rd slot still produces this issue.

You are correct, as I used just one module for a long time. Now two.

The hum is absolutely dependent on the gain of the module. Interesting about the sinusoidal pattern. Are these smoothing caps possible right next to where the modules are plugged in? I don't like 500 chassis designs like this with components so exposed on the front side. Blindly putting in a module is difficult, and you can easily bump the caps there. Perhaps I did.

So, to answer -

A - Yes - hum varies with the level of module gain
B - Yes - the dynamic mic works fine, with the added module hum.
C - No - phantom power on the first unit does not affect the hum of the second unit in slot 3.
D - Not sure here. I had taken the input XLR out of the chassis completely, leaving the output going to TRS input of my converters. Would this impedance load work with a second unit that is still off? AKA the outputs of my Sytek preamp while powered off?

This point out to the powersupply of the chassis then. Either make it check or send it for repair, if it's still under warranty. Unless Boswell can be more specific on what needs to be changed.

Do I gather from what you say that if you listen to the output of a module in slot 3, the hum level is independent of whether or not a module in slot 1 is providing phantom power to its microphone? This is a critical point, and, if true, rules out a voltage-dependent leaky capacitor as a cause of the hum.

I found the only email I could on Lindell's site but haven't heard back in a week now... eh...

You are correct - the hum level of module 3 is independent of module 1's phantom power.