Sounds like the relay is turning off and on, and the fan doesn't get a chance to get started. Take a look at the signal you are using to turn the relay on. My money is on a low DC voltage with higher voltage noise "spikes" on the line. The spikes in the signal are turning on the relay momentarily. You'd need a low-pass filter on the switch, which is essentially a small capacitor in parallel with the switch. If you're using a DMM to look at the signal, the Vdc will filter out the spikes, but should be able to see the dc voltage. Switch it to frequency measurements to see if you're picking up an AC noise signal. Good time for an oscilloscope, if you've got one available.
Oh, and if your ground wire is exceptionally long (i.e. a small antenna), you may be picking up the noise there. Check the ground connetion and keep the wire short.
I used to be an Applications Engineer for Tektronix.
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Dangerous Doug
"You're kidding, right?"
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