97 840 70,000 mi. Runs smooth, no open door or hood shake. Feels like it's making power OK. Problem is... it sounds like a sewing machine. No check engine light indicated, no smoke. Changed plugs, cleaned things up, changed oil. Trying to avoid taking it to the shop of course... any ideas?

Check belts and tensioners. My old car sounds similar and I had a bad tensioner which then ruined my timing belt.

Thought I'd update this old thread and le you guys know what the issue was...
I called my mechanic and gave him the story. Final result was that not driving the car caused the lifters to bleed down, he said to drive it for a few miles and see if it went away which it did. He told me that the lifters can take awhile to pump up. I had started the car but was afraid to damage it with the clacking lifters. Sounds dumb but the car is not my daily driver.

Thanks for the update. I only have one lifter that taps when cold but quiets as soon as it warms up a bit.