Based on my experience with DNA contamination (different problem but can be due to similar causes), there could be a number of potential issues. First, how clean is the area you work in? Are you working under a hood or on the open lab bench? Also, how is your pipetting technique? It is possible that accidentally touching, even briefly, the edge of a tube or other surface can introduce contamination. Another possibility may be that your reagents are contaminated. I work with DNA primarily in a clinical genetics lab so whenever we have some sort of contamination, it is typically due to one of the above issues which could also lead to RNAse contamination.Hope this helps!