Yesterday, during a torrential downpour, I heard a loud gurgling noise in the kitchen. Turns out there was a 2-3 foot geyser shooting up from the kitchen sink drain. It smelled like onions, but not sewer gas. I put a plate over the drain, thinking that it was just storm related one-time deal. But the intermittent gurgling/sound of rushing water has happened 4 more times. On a hunch, when it happened this morning, I went outside to listen. My next door neighbors were doing their laundry. (Their washer is on their back porch.) The water doesn't appear to be human waste. Not to be gross, but there's no "particulate" matter. The sink drains normally. Do I call a plumber? Do I call the county? Do I just wait for Tropical Storm Hermine to pass? Any insight will be very much appreciated. Also, this is not an apartment building. These are single-family homes.

i'm guessing there is a sewer pump tied into a common sewer shared by you and neighbor/s?
also that there is a clog in that sewer pipe so when the pump comes on some of the water is coming back into your pipes.
it'll take some detective work to figure this all out.
possibly the city/town has a drawing of the sewer arrangement in your and your nieghbors yard?

I'm sure there is a pump/shared line and that the county has plans. The homes are about 30 ft. or so from the road and about 15 ft apart, if that's relevant. Should I let the county know? Or (grimace) call a plumber?

at very least you need to know location/s of any piping, connections and cleanouts.
hopefully the county has that information and can supply you with a copy of the map and measurements.

once you have that you can call a sewer cleaning company with a good reputation.
you can save yourself some money by looking around, doing some measurements and if necessary uncover any buried cleanouts.
you can also find out who else shares that pipe. that info should be on the county's plan. then contact them to tell them of the problem and also tell them they won't be able to use the water while the sewer company has the pipes open. neither you nor them want 'old faithful' erupting while they are working.

What city are you in?

The plans should be available from the city's building records department unless you reside in an unincorporated county, in that case it would be at tge county's buildings department.

If these are single family homes how can they share a line until its beyond their respective property lines. I am quite sure down where I am you can't have your line under your neighbors property. Sharing a pump seems weird.

Is this geyser coming up on other drains? Any sign of backup in your shower, tub, gurgling toilets etc?

Is it possible its your dishwasher pumping discharge into your kitchen dispiser and you have a clog there instead?

This is my elderly father's house in Bradenton. Several homes share a line on the right-of-way leading back to homes built behind my neighbors' home after a lot split. In talking with them, situations similar to this but less intense have happened numerous times in the past. The gurgling/rushing water noise is now happening over a dozen times a day that I know of. The actual geyser only seems to be happening during intense downpours. My father, bless his heart, hasn't heard a thing this whole time and was completely unaware of what was happening. After the storm passes, my neighbor is going to attempt to clear the blockage. He seems to know exactly what to do. BTW, this neighborhood apparently has a reputation for less than stellar (crappy!) plumbing.