What is a symptom of the piston striking the head?

[Gnarlodious]

I have a '77 Rabbit 1.5 engine 56,000 miles. I put a new rebuilt head on it less than a thousand miles ago, it has a 4-notch gasket. Compression is excellent, has great power, starts easily. Recently it has set up a sharp knock that sounds a little concerning. The knock goes away when the engine is hot and at cruising speed. It is intermittent but may partially return when the engine cools a little. I have rotated the crank manually but no indication a piston is striking the head. I've driven it maybe 100 miles with the knocking. Am I on the right track with this diagnosis? Does it sound like I need to take the head off for inspection? Am I safe to drive it until I can fix it?

[fatmobile]

Maybe it's an injector.
 Does it knock when you step on it then go away when you let off?

[Gnarlodious]

Maybe it's an injector.
 Does it knock when you step on it then go away when you let off?

No, it knocks mostly when it's cold and below operating temperature. Seems to be worse under no-load conditions. I would almost think it is piston slap except only 46k miles on this block. Knock goes away or is greatly reduced upon continuous cruising.

I've ordered a 5-notch head gasket for it, so will report back when I know more.

[fatmobile]

 When the engine was rebuilt,
 did it get new piston rod bushings and get them reamed?

[Gnarlodious]

When the engine was rebuilt

I wasn't rebuilt, original 1978 engine. My grandfather's car. I put a reconditioned head on it last year, bought from The Parts Place. But evidently that head was shaved as far as it could go.

[sgnimj96]

I'd pop-test the injectors before removing the head.

[Gnarlodious]

Has new injectors and a rebuilt pump.

[fatmobile]

 I'd still test the injectors.
 New injector inserts aren't as good as they used to be.
Could be a pisser,
 or could be the rod bushings on the small ends of the rods.

[Jetmugg]

Milling the head will have no affect on pistons hitting the head.  The head surface is flat, with no built-in "combustion chamber" of any type.