Take a video of the noise if you can. If its a loud crackly ratter under acceleration i would suspect the tensioners or wristpins. And there really not that hard to do. but takes around 14 hours. and 1 1/2 cases of beer Wristpins would be a engine removal. so probably quicker

 

I'll try to get a recording of it this weekend. I can't hear it while driving the truck. Only from around the wheel well or underneath.

 

What Krewat said plus check all the heat shields for broken welds, then try my tried and proven method for finding noises. Remove the belt and start the motor cold and see what happens to the noise, this will eliminate all the other noise from all the mechanical stuff bolted to the motor and the belt itself. Remember the A/C compressor, power steering pump are mounted low on the motor and the alternator is located where the cam used to be and noises can travel when they are bolted to the block, you can run the motor for a short period without doing any harm.

In all the motors I have pulled apart (not a V10 among them) I only found one bad wist pin and I didn't think that was the problem, it was a 73 Triumph Spitfire and the pin walked out and made a grove in the cylinder wall. The one I thought was a bad pin was a cylinder bored so far out of alignment the block was junk, 79 351M.

Denny

 

If all they did was crawl under it and listen to it for a couple minutes they didn't diagnose anything, too many shops rely entirely on computer diagnostics and there are fewer real mechanics these days that I can diagnose problems without their computer.

Is it a ticking rattle or a constant rattle? I've heard of torque converter's having internal parts make noise and rattle and still work? I've never actually heard one that has done this but it would be a low-end engine noise like you described.

 

If it was a engine noise from the wrist pin or connecting rod bearing disconnecting the sark plug coils one at a time should significantly change the sound when you hit the one that's making the noise. This is true for a leaky exhaust manifold gasket also..... Sound travels in interesting ways....

 

After letting two more mechanics check it out they both think it's piston slap on the number four cylinder. They said its annoying but the engine should hold up for awhile. What do you guys know about piston slap? One mechanic said switching to a heavier oil may quiet the noise a little.

 

The only real thing heavier oil is good for is short-term help with low oil pressure or heavy oil consumption to get the last few miles out of the engine and not really a fix for any mechanical noise is not related to oil pressure.
"Piston slap" is not something I have ever experienced on any Ford engine I have been around or owned but my neighbor has a 02 Chevy 6.0 that sounds like a crew of guys are beating the bottom of the engine with sledgehammers every time it's cold started and I know that it has 15W40 Rotella oil in the pan now and the mechanics say it's normal piston slap.
I'm not completely convinced that piston slap is the answer to your noise.... yet.