Faucet only leaks when running

Newly installed kitchen with sprayer No-name, not the cheapest on the display, but < mid-price
Opened up the valves after install, listened and watched for leaks for 2-3 minutes, absolutely nothing! Turns out I celebrated too early. Ran both sides, tried the sprayer, cleaned the crap out of the screen, all good. Turned the handles off and heard the drips, did a quick newspaper test and pinpointed them as coming from the cold handle. Tightened up the connector there, but that's not it. Only a small amount, and only when it's open, and it's not slop splashing from up top.
Where am I going wrong? Thanks in advance for any help.
--
The real Tom Pendergast [ So if you meet me, have some courtesy,
aka I-zheet M'drurz [ have some sympathy, and some taste.
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I-zheet M'drurz wrote:

It's a 2-handle faucet (like Delex)? And the supply fittings are not the source of the leak? Could it be a fine mist from the spray wand fitting underneath that you can't see? If not that, then it's a flaw in the faucet casting or else the Cold stem is leaking around its O-ring.
Jim
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Speedy Jim wrote:

Yes.

No, all is tight, left a catch pan under there overnight, dry ar a bone this AM.

OK, that's worth a look.

And so are those. After I rule out any possible interaction from the sprayer, I'll pop off the handle and try to make sure all is well with the stem/cartridge or whatever's under there.
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The real Tom Pendergast [ So if you meet me, have some courtesy,
aka I-zheet M'drurz [ have some sympathy, and some taste.
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