Low pass LC filter goes "wrong" with capacitive load
Welcome to the sad realities of doing real electronics work.

There's two possibilities (one of which I missed at first).  Frankly, I'm not sure which is the problem.

The first problem is that your filter impedance is roughly \$\sqrt{\mathrm{\frac{220\mu H}{100\mu F}}} \simeq 1.5\Omega\$ but you're terminating the filter with 100 ohms and 6.8k ohms.  So even with ideal components, the filter response will be very underdamped.  This is probably not the problem if you have ideal components (edit: it's not the problem -- the resonances are there, and they're strong, but they're between 1kHz and 2kHz), but it would show up if you analyzed the filter response or simulated it.

The second problem is that with those component values, you may be having issues with parasitic effects: when you design with any cap, you think you're getting this a nice pure capacitor, like you write down on the schematic.  But you're really getting this infinitely complicated thing that may act like a cap in series with a resistor, or in parallel with a resistor, or in series with an inductor, or all of the above, or -- as you push into higher frequencies or lower impedances -- ever more complicated stuff.

Aluminum electrolytic capacitors are made by taking a couple of strips of aluminum foil, layering them with paper soaked in salty goo* and winding them into a a cylinder.  When you use the cap outside of its intended frequency range, the fact that there's a coil of conductors involved, it starts to look inductive.

Similarly, inductors act like inductors in parallel with capacitors (because the wires are spaced closely together, and have winding-to-winding capacitance).

That's a pretty big cap to be using in the presence of a 100kHz signal; I suspect that if the filter as designed doesn't have a resonance at 100kHz, then the parasitic elements of your various parts (coils + caps) is causing a resonance.