Extra Ignition Click

[TractorZoo]

My 2004 L3430 started making an extra click when keying on the glow plugs. It's as if I turned the key back to off and then back on again but really fast.
It used to be a single click but now two. Same when keying off - an extra click that had not happened before.
Other than that, normal glow plugs and starting.

Any ideas on what is about to fail?

 

[TractorZoo]

Update

After a week of not being used the battery voltage is 12.4V.
Ignition/relays sounded normal today - no extra click.
Measures 14.4V with alternator.

I will chalk this up to a weak/old battery (9 years?).
The tractor had sat unused for months during winter and I noticed this issue upon the first few short uses.
The last use was for a little more than an hour so I think the battery had enough charge to start normally today, despite being low.