My 280 has a very pronounced hollow, rattle/knock noise coming from the lower chain area, somewhere behind the harmonic balancer. Oil pressure is good, compression strong, pulls hard - should I be worried?

What chassis & engine? Year? Miles/hours?

I my 280 SEL 4.5 it was the chain tensinor

Interesting re tensioner - is it a tough job?

The car is a 1968 280S, M130 motor, 88XXX miles, I think 108.016 chassis.

First, check thoroughly that the dampener disc is not loose on the crankshaft front hub, or that the rubber in the dampener has not deteriorated.

If those are OK, you may be hearing a loose or broken timing chain lower guide rail, or the sound of a slack chain. The chain tensioner assembly of the M130 is mounted in the cylinder head immediately below the surface on which the cam cover seals. The tensioner assembly is composed of the tensioner sprocket, a rocker on which the sprocket is mounted, and the spring/hydraulic tensioner itself, which is inserted from the outside of the head in the right front corner.

That noise is always concerning and should be evaluated immediately. Frank has provided all the possible causes. Failure of any timing chain components can be catastrophic

__________________
Ron
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Broken lower guide rail sounds concerning. Does the engine have to come out to inspect it?

The chain appears tensioned after the engine has run, but goes to ~1" slack once it has been sitting for a day or so.

I'll start by prying around the rubber front disc and see if that's the culprit.

Hi,
No need to remove the engine to replace the timing chain guide rails.
You will need to set the engine timing to zero degrees, pull the timing chain to one side, remove the radiator, remove the distributor housing, power steering pump and remove the crankshaft damper. You will see the guides inside and they are held in place by two pins. These are threaded on the hollow section and can be removed by making a home made pulling device by using a bolt and some spacers. Make sure the threads are clean before you start threading your bolt in.
This is a tight space to remove and install new guides, especially the lower one.Replace all 3 guides as they are inexpensive. If your chain is worn you should replace it as a loose chain will slap against your new guides.
Reinstall everything you removed. The distributor will need to be set so that the rotor button points to a notch in the body of the distributor.
You will need to check ignition timing with a strobe light.
Good luck, this will occupy you over a weekend.

Tassie Tiger