Need some help please !

[c10sob]

Ok , so for the last year on my first gen chevy 350 engine compartment or engine noise that has drove me crazy. Ive tired to turn up radio and ignore it but i am into fixing the problem not masking it . I actually started a list of things i took off and dampered to find the noise , at first i thought it was a brake line/ fuel line rattling . nope. maybe the spring from hood, nope . even gone as far as removed fenders, hood, grill , air cleaner, wipers, wiper gears,cowl . thought it might be valves because it only happens at 2000 rpms under load when crusing around 2000 - 2500 rpms. Ive adjusted them with truck running acouple of times , everything looked good and moved good. i thought if the valves are making noise they would do it all the time not at a certain rpms. what do you guys think ? could it be a vacuum leak or ignition problem ? My truck is a 1966 with a first gen 350 . Any suggestions would be great to look for . I did notice that my valve covers were short and i had locking tall hardware on the rockers . i have no idea , i am guessing at this point.