Why does my oil-filled radiator tick and ping all night? Is that normal?

The ticking is normal, so you didn't get a dud. Oil-filled radiators are steel full of hot oil, and when they first heat up the metal expands and every joint and panel shifts a little. Mine does the exact same thing for the first half hour after a cold start, then mostly settles down once it reaches temp. If yours pings every few minutes all night, it's prolly cycling on and off as the thermostat kicks in, which starts the expansion process over each time. A couple of cheap fixes to try before buying anything. First, turn it on an hour before bed at full power, then when you get in bed drop it to the lowest setting. Less cycling means less ticking, and the residual heat in the oil keeps radiating for a while even between cycles. Second, if it doesn't have a built-in timer, a plug-in timer outlet from any hardware store works. Set it to shut off around midnight and come back on at 5am so you're not getting up at 3am to freeze or fumble for the switch.

Honestly though, for sleeping in a small room, the better play might be to stop heating the air and heat the bed instead. An electric blanket or a heated throw under your covers keeps the room cool, which most people actually sleep better in anyway, and there's zero noise and zero cycling. I ran my radiator just to take the chill off before bed, then shut it off entirely and let the blanket do the rest. Took me one winter to figure this out and I wish I'd done it sooner. On mini splits, the quiet reputation is real but it applies to the inverter-type units. Those don't click on and off the way space heaters do, they just ramp thier output up and down to hold the temperature, so the noise is a steady low whoosh instead of a startle every twenty minutes. The catch is cost. Between the unit and getting someone to install it properly, you're into real money for a 12x12 room. If the ticking and cycling is the only complaint, a timer plug and a heated blanket costs about a tenth as much and might fully solve it.

One thing that helped mine was just moving it off the bare floor. Mine sat on hardwood right next to the headboard and every little tick came through like a drum. I slid a cheap cork mat under it, the kind sold for washing machines, and the sound dropped way down. Turns out most of what I was hearing was the floor amplifying it, not the heater itself. Might be worth trying before you spend anything, costs almost nothing to test. Also try setting it a couple degrees lower than you think you need and adding a second blanket, the cycling gets way less frequent that way.

Still pretty new to actually thinking about heaters so bear with me, but that timer plug idea caught my eye. I use a basic mechanical timer for a lamp and it hums a little, and I've read some of the cheap ones aren't rated for anything that pulls a lot of power. Is that a thing with heaters too? I'd hate to set one up and then worry about it all night, which would be its own sleep problem. My situation is slightly diferent since I'm in a rental with baseboard heat, so no mini split options for me anyway. I've just been running a small ceramic heater on low instead. It still clicks when the thermostat kicks in but it's way less than the oil one my buddy has.

That heated blanket idea is tempting but I'd be nervous leaving one switched on all night while I sleep. Mine has a setting that's supposed to turn itself off after a while, but I never fully trusted it, so I ussually unplug the thing before dozing off. Maybe the newer ones are better. Does anyone acutally run theirs through the whole night or is that something you're not supposed to do? Because if the answer is no, then I guess you're back to needing the room itself warm, which puts me right where the original poster is.

I do residential HVAC, and the split question is easy to answer from expirience. An inverter unit in a 12x12 room basically idles once it hits setpoint, usually down around a third of its rated output, and there's no relay clunk or expansion ping because nothing is switching on and off. The catch is the indoor fan never fully stops, so it's a constant faint airflow noise. Fine for most people, but if you're the type who hears every tick in the dark, worth hearing one running in a showroom before you commit. The oil radiator on low all night would cost maybe fifty cents more per month than the cycling approach, and steady gentle heat is easier to sleep through than on-off anyway.

Update: the cork mat suggestion worked. Moved the heater off the hardwood onto a cork mat from the hardware store and the ticking went from drum-tap to barely there. It still ticks a bit during that first warm-up, but it doesn't carry through the floor anymore and I've slept through it all week. Also put it on a cheap timer so it shuts off after I'm asleep instead of me doing the 3am shuffle.