voltage gauge/cycling noise

[moel]

jumped into truck at lunch time, started up and idled for 2 minutes(ambient temp=39°), heater controls: fan=off, temp=mid range, location=at feet. My question is what is cycling sound (coming from right side of engine compartment, close to firewall) also noticed that voltage gauge registered just under 14 rather than just over 14(normal). When returning to work, started up and idled for 1 minute, noticed same events happening as before so i increase idle speed from normal 750rpm to 1000rpm and gauge returned to its normal position. 2266 on the clock. Not worried - just curious?

 

[bpenrod]

Sounds like the grid heater cycling. I know mine will pull it down below 14 volts when running and when it shuts off it's like a kick in the pants!