I shut my car (99 E300) down yesterday and noticed a really strange sound coming from under the hood - sizzling, popping, hissing, ticking.

Finally tracked it down to the windshield washer fluid pump, which must be starting to leak a bit of air (unless this is normal behavior!) I'd just used the washer fluid before pulling into the garage, so this makes sense. It stopped after a few minutes.

Any ideas? Cost and ease of replacement, if needed?