merging GeoTiffs with GDAL causes grid-like artifacts

I’m processing ASTER DEM tiles, merging them into larger blocks (to minimize visible tile edges) and importing the contours into a postgis database. Workflow:

gdal_merge.py -of GTiff -o merged.tif tile1.tif tile2.tif
gdal_contour -i 10 -a ele -snodata -9999 merged.tif contours.shp
ogr2ogr -f PostgreSQL -nln srtm4 -append -t_srs 'EPSG:3785' contours.shp

For some reason the resulting “merged” contours have grid-like artifacts:

When I skip the merge step and process the same two GeoTiffs separately, there are no artifacts. I’ve tried merging with gdalbuildvrt and got the same grid effect. Why is this happening? How can I avoid this? Thanks in advance.