Vehicle - 97 f150 2wd 4.2 148k miles. I drive the truck 1 to 3 days a week. I drove the truck on a Monday and no issues. 4 days later on Friday I start the truck and it starts "clacking" on the passenger side, very constant and RPM related, much like a loose rocker arm. Good oil pressure and everything was as it always has been. I shut it off after about 20 seconds. Next day I did a compression check. 180-190 on all cylinders except #1 which is at 140. All cylinders have psi with no bleed off. I'm thinking that its a dead lifter, or rocker arm/pushrod. Specifically intake - it's not opening enough/long enough to get the full stroke on the intake cycle. I pull the valve cover and nothing out of the ordinary. Decision time - pull the intake? Well I did and again nothing oblivious. I went ahead and replaced all the lifters and reassembled, and it didn't change a thing - the noise is still there. I don't think its a rod/bearing as I have good oil pressure. Don't think its a piston because I have constant psi. Any thoughts? Thank you in advance.