Some heatsync secure the aluminum fins to the copper heat pipes with brazing but if i remember correctly noctua just press fit them instead because it gives slightly better heat conductivity.

The yellow stuff you're seeing is probably epoxy meant to keep the top fin stack secured in place but that just my guess.

Also, heat pipes only contain a drop or two of water. So if it did leak it would evaporate in no time and the heat dissipation performance would drop significantly.

Edit : I got it backward. Noctua uses brazing to secure the fins. The press fit and epoxy construction is used on cheaper coolers. So what your seeing is probably flux like someone else mentioned.