My 91 4.9L makes a slight "knocking" noise when at idle which goes away above 900 rpm. It's very similar to the clatter of a diesel engine. After the truck warms up it's not as noticable but it's still there. I've heard this noise in the past following an oil change but it would always clear up. My last oil change was about 300 miles ago and is always done at 3000 mile intervals with 10w-30 oil. I currently have 95,000 miles on the truck. I've removed the serpentine belt and inspected all the pullys and belt which all seem to be in good shape.
Lately (because of sky-rocketing gas prices), I've switched to 87 octane fuel. Could this be the cause and wouldn't I notice it while driving also? The engine runs great!
I'm hoping I don't have to start shopping for a new motor.

 

I've driven 300-powered trucks most of my life, and they characteristically make knocking sounds when first fired up.
1. Hydraulic lifter clatter, which sounds distressing, but then again my current '91 has clattered horribly on startup--for all of its 178,000 miles.
2. There is another knock in there, a subtle noise, that I wonder about, same as you describe. All I can say is just drive the thing and hope for the best.
Eddie

 

The 4.9 does characteristically have that knocking noise ...some worse than others . What you can do is remove the oil presure sending unit and screw in an actual presure gauge and verify you have 40 to 60 Psi at idle . That engine has seven main bearings and I believe the clearances are somewhat greater than other engines . Also Ford has a revised oil filter insert that contains a spring and check ball that keeps all the oil in the oil filter which eliminates the start up knocking noise ... Ford Part Number E4TZ 6890 A .
Paul