Hmm Zammad thems to be a bit hungry 
Could you try to set those values in your environment (make sure they survive a reboot)
export RUBY_GC_MALLOC_LIMIT=1077216 RUBY_GC_MALLOC_LIMIT_MAX=2177216 RUBY_GC_OLDMALLOC_LIMIT=3000100 RUBY_GC_OLDMALLOC_LIMIT_MAX=4000100
This tells the ruby garbage collector to clean up more often, within the given limits in bytes.
P.S.
Which Zammad version do you use?
could you provide a top / htop output sorted by memory consumption?
regards
Johannes