toilet makes other toilet flush

Situation:
I flush the upstairs toilet. It flushes just fine.
About 3 minutes later, the other toilet downstairs does the end-part of a flush. I can hear the valve downtsairs close(makes a reassuring "thunk" sound).
Did a little experiment. Flushed upstairs toilet, ran downstairs and watched the other toilet. The water level in the tank goes down a bit, and it of course flushes back to normal level.
Any remedy for this?
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Spitfire 1500 wrote:

Clear the obstruction in the vent pipe. Other possibility that I can think of would be some kind of problem with the flush valve.
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I think i just figured it out. The downstairs toilet has a higher-end 400 series Fluidmaster. Looks like the filler hose went too far down into the fill tube. Looks like I'll get an adapter.
Also, this is a very infrequently used toilet.

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Spitfire 1500 wrote:

It's not really "flushing"; the tank is merely refilling itself and you hear the sound of the water going into the tank. So it's not a venting issue.
If you have the FluidMaster ballcock (fill valve), the one with the black plastic float that rides up and down the "standpipe", they can be very sensitive to changes in the supply pressure. The kind with the float on an arm made by Coast Ind. is also sensitive.
In either case, replace or rebuild the ballcock.
Also check that the small "refill" tube does *not* extend down into the overflow standpipe. If it is down inside, self-siphoning of the tank can occur which will result in periodic refilling.
Jim
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