My LS has an intermittent chirp at idle. Is that a lifter, or could it be an exhaust leak?

It could easily be the exhaust. A leaking exhaust manifold gasket or a cracked manifold can sound a lot like lifter tick, especially at idle and on a cold engine.

Check the exhaust first — it's cheaper:

• Look for black soot around the exhaust ports and manifold flanges.
• Check that the manifold bolts are tight and the manifold isn't cracked.

Then check the lifter side:

• Listen at the valve covers with a long screwdriver used as a stethoscope. Lifter noise comes from the top of the engine.
• Check the oil level and pressure. Low oil or low pressure is the most common reason hydraulic lifters don't pump up and start ticking. An overfilled crankcase can also aerate the oil.
• Fresh oil of the right grade often quiets a noisy lifter. If it doesn't, pull the valve covers and look for damage.

Don't ignore a persistent tick: a failing lifter can wear the cam lobe, and pieces of a failed lifter can damage the rest of the engine.