Quiet Knock and RPM Jump

[mmccrack]

Hey all, sorry to make this my first post, but I can't seem to find anything online or on the forums that seem to match my specific symptoms.

I have a 2004 Outback Legacy with about 110k miles on it.

About a week ago the car started to make a horrible screeching sound and the AC suddenly stopped working. I took it into the shop and they found a loose bolt on the timing belt tensioner which I had them replace. That fixed the AC and screeching sound; however, since I've gotten it back, I've started to hear an odd intermittent sound.

I don't really know how to explain other than it sounds like someone very very very softly knocks twice on the passenger side and then the car jumps up about 500 RPMs. It will happen every 10-20 seconds at around 20-35 mph, but not every time I drive.

I've seen plenty of posts about rod knocks and piston slaps, but from what I can tell it doesn't sound like the same issue.

Anyone ever run into anything like this before?

Thanks ahead of time!

[1 Lucky Texan]

is the check engine light on?

how old are the spark plugs?

[mmccrack]

The check engine light is not on and the spark plugs are about 6 months old.

I just took it out for a few minutes and I noticed that it sounds almost like something is stuck. Like a rubber suction cup is sticking which also coincides with hesitation. It's only happening when I accelerate.

It also just made an unusual sound like you might hear when turning the car is running low on power steering fluid. I could feel it through the pedals. I'll check the steering fluid in a few minutes (though I think it's good on that and I wouldn't think that would cause any hesitation).