I have heard reports of this when the copper washers have not been changed. The copper washers are the seats for the valve spring, and the spring wears a seat into it. When removed for DV seal replacement, the spring ends up in a different position.

I replaced the seal and washers in my car a few months ago, and it also sounded louder for the first 200 odd km, and then the noise mysteriously went away.

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'98 E300 DT
'87 190D 2.5 turbo RIP
'92 S500 Sold
'03 3.2TL-S A-Spec