Small clatter in eng. after change to syn oil

[Al S]

Last year I had my 4cyl eng rebuilt. I ran dino oil for 10K then changed to
Mobil 1, 15-40, in keeping with Audi recommendations. Now when I start then
eng. I get a small clatter coming from the lower eng. rear. Sounds like
somebody rolling dice. It is not a loud banging and the sound goes away
after about 1 minute and doesn't return until a cold start. Morning temps
have been in the 30's. Is this my oil pump? Is the syn oil too thin? Audi
has some really weird recommendations for it's oil requirements. I usually
use 10w-30 in all my cars/trucks. We experience temps from 0-100f thru the
year. I don't get any noise from the valve train.
What oil viscosity do you guys recommend?
Thanks
Al Streicher
1990 80, non-q, Black 2.0L 4cyl.
Thanks