There are a couple otther things that have come to mind.

The age of the belt could play a part in tension, meaning if it is old and stretched it could cause less tension due to length.

Like Bill said, check connections

And on the 16V, I assume this was according to the gauge in the car. Most of us have seen our own gauges as semi inaccurate when using a scanner. The 16V may not actually be that high.

Otherwise it seems as though the tensioner may be getting weak, as you said..it was already diagnosed as not being up to par. The belt could be slipping.

[quote="BillBoost37"]

And on the 16V, I assume this was according to the gauge in the car. Most of us have seen our own gauges as semi inaccurate when using a scanner. The 16V may not actually be that high.
quote]

The 16 volts has to be checked at the battery with a regular voltmeter to see how accurate the dash gauge is. It'* probably off or your battery would have lost its water by now with overcharging.

It'* a brand new battery, and a new alternator, so how would I be able to tell if the alternator'* bad? Just checked the connections on the battery, they're clean and tight.

I'm beginning to suspect the new alt is bad, but I still think the belt is slipping as well. It only hops up to 16 for a few seconds, then drops back down to just under 14, and only when I have headlights and defrost on.

I'm going to do the tensioner this weekend just to eliminate it from the list, since I know it'* about shot. Maybe it will solve all of these problems, if the belt is slipping.

Did the tensioner this weekend, and all my stray noises and vibrations seem to have gone away. This seems to have fixed the problem, I think I had a bad pulley on the tensioner in addition to a weak spring. It runs much quieter than it did, too, can't even hear the engine at highway speed, when before it had quite a hum to it.

That'* great. Did you happen to snap any pics? We love those pics.