I know that there aren't a lot of you using hydraulic lifters, but I figured that this was the best place to post this question...
I recently put a new cam in my engine (it's the Comp XE-HL .564" lift hydraulic grind) and went with Comp's Pro Magnum flat-tappet lifters. Ever since I fired the motor for the first time, I've been getting a lot of valve train noise. I've checked to make sure I have the correct pre-load three times now, and everything checks out. Nothing was changed other than the cam and lifters.
Are the Pro Magnum hydraulic lifters typically that much noisier than Comp's regular lifters?

Did you break-in the cam correctly? With fresh oil, lube, run time, etc.

I used Comps break-in lube and their break-in oil additive along with Valvoline's race 20W-50 race oil. The cam has about 1 hour of operating time on it (including 20 minutes break-in time), and I'm planning on changing out the oil this weekend. The cam doesn't seem to be going flat...

Could be your valvesprings are too stiff. Depends on what cam you had before, and the belonging springs...

What method did you set the preload with? Those lifters have different setting procedure than the normal ones, try 1/8 to 1/ 4 preload instead of .002 lash

cam and lifters are not what they used to be

The springs are whatever Indy put on my heads before they shipped. The receipt has "911" on it (indicating Comp #911 springs, I guess), but they're double springs, so I don't know exactly what they are. Indy claims they're good to .650" lift. The previous cam was the Comp XE-HL .545" lift grind.
I set pre-load by going 1/4 turn beyond zero lash.

I have the same issue with my street car. Hyd Cam and Eddy Alum Heads. I have wondered if as the heads heat it effects the lash but you would think a hyd lifter would compensate for that to a point. I will watch this post with interest. Hopefully, someone will have the answer we both need. I am thinking about putting in a solid. If I am going to have the racket I may as well reap the benefits of the solid.

My new build has the same sound. I could never find the cause or reason, but people have told me not to worry about it. I've now got about 1,000 miles with no problems so far.

with aluminum heads 1/4 turn (.010) of pre load is not enough. should have been 1/2 to 3/4 iron heads and 3/4 to 1 aluminum? I would add 1/2 turn pre load and if it still ticks 1/4 turns till it is gone.

My Eddy-headed 446" RB made valvetrain noise, fairly fresh motor, after I'd tightened the valve cover fasteners a bit. Turned out to be a few rocker arms tapping on the valve cover baffles and the rocker stand clamps coming in contact with the valve covers and transferring sound. It was the stands at front & rear. A few minutes scientifically clearancing the baffles with a punch & hammer (protect the outside of the covers) and filing the outside corners of the stands cured it.
If that's not it, try another 1/8 turn on the preload, though I've never run more than 1/2 turn.
This assumes your geometry & pushrod length are correct.
20W-50 is a bit heavy for my tastes on anything less than 200,000 miles or below 40 degrees F air temp.