i dont no if this will help but my 6 was maken a sound and happen to be a lose
spark plug

 

is it the lifter rattling? if it is, you will usually only hear it when accelarating or when lugging the engine. just about every 300 does it.

 

Mine has the same prob. its an 82' w/a hooker header on it. the head ahs been P&P and ive rechecked the rocker arms. Im awaiting putting on a intake and holley 390 so maybe it'll fix the prob. But I think it might be my emissions stuff, but i only heard that from some guy at a meeting once.

 

The EGR tubes are known for cracking. I would pull it off and check it thoroughly. The usually split along the length of the tube on the verticle portion.

 

I have a 94 300 six and I had the exact same problem, it idles and cruises smooth and quite. Under load however there was a ticking sound when going up hills or accelerating on the passengers side like a diesel would sound like.

At a stop I use to put it in drive and put on the bakes and and give it gas for people to here the ticking,just reeving the motor in neutral will not make the ticking noise it must be under load.

Heres the strange part, I lent the truck to my friends wife when me and her husband went fishing for a week with his truck so she could get around and go to work. When I got the truck back the ticking sound was gone!!!! Its been 5 months now I dont know what happened to the ticking sound. My Friend says his wife has a lead foot so maybe going down the road at 90 mph did something I just dont know. A sound like that is a tricky thing to find I wish I could help but I hope my story helps you track it down,there may be no new parts involed with it.

 

try using a section of heater hose to listen at the valve cover, see if you can hear an increase in sound when closer to any one rocker, if not also use it behind the engine and along the exhaust and air pipes to see if the sound gets loud. this should help locate the area the sound is coming from

 

Mine was the AIR tube. $20 worth of pipe fittings fixed that.