Just the other day a new, slight rattle, actually more of fast tick almost like a fan noise or maybe even a belt started, seemingly from behind​ the dash or firewall. Seems to be intermittent too, though more there than not. Not too loud, others may not even hear it but I do. No A/C and it occurs regardless if I have the heater fan on or not. I reached around behind the dash and felt nothing lose, and you can't hear it from the engine bay. Actually, its so quiet you really only hear it at idle. But it's new and it always concerns me when something changes. Any ideas?

 

Take the belts off and briefly run the engine and see if it disappears. If it does then put the belts on one at a time to isolate. If it doesn't go away keep looking.

 

You don't mention the year make or model so I'm going off your avatar...Looks like 1970. Do you have a mechanical divorced voltage regulator mounted on the firewall in the engine bay? Those will make funny noises when they are on their way out or it doesn't like the charge it's getting from the alternator and or if there is a bad ground or other bad/lose connection in the charging system. Bad battery?
Get a long screwdriver or something long and metal and put it on the regulator then up to your ear. A Mechanics Stethoscope works well too.

 

Thanks for the input everyone. Yes, '70 droldsmorland - and the battery's brand new. I'll bring these ideas up next time Im in the shop, hopefully before something goes terminally wrong.

 

You could have a baby exhaust leak, although that should get louder with RPM. With a cold engine, start it, and go feel up the exhaust manifolds at their inlet and outlet, as well as the pipes down as far as you can reach. Do not get near the fan, and obviously stop once the exhaust warms up. You're feeling for little pops of exhaust gas coming from somewhere. Alternatively, you can jam a smoke bomb up its butt and cover the tailpipe with tape and hold it on for the main thrust. Smoke will come out a leak (this is done cold and off.)