It's sitting at about 125k, and define "tune up"......

I don't take it in for a whole lot of work, unless it'd be cheaper for
the shop to do it, or I don't have the tools for the job.

I change the spark plugs every 3 oil changes (I'm experimenting to find
which gives best economy) but I tried switching out the spark plugs
from Platinum +4's back to standard plugs when I first noticed the
sound, but that dropped performance significantly, and made my engine
noisier (I have no idea why, doesn't make sense to me that it should
get louder with standard plugs).

I changed the plug wires at the turn of 100k, but I thought they were
good for 50k or more....

I change my oil religiously at 3k (It's just my thing, probably don't
need to be so anal about it), and I use full synthetic, and a midgrade
oil filter (Fram tough duty), the air filter I change less religiously
(every 3 oil changes or whenever I think it's getting bad)

I tried a can of Seafoam (1/3 in the oil, 1/3 in the gas, and 1/3
through the PCV) just to see if I could free things up. Did wonders
for my injectors, even boosted my mileage, but the noise persists.