If you have followed the cleaning instructions, a metallic off-flavour will not be due to the fermenter.

If you take care of your equipment, 304-grade stainless steel does not impart metallic flavours. This is most likely an issue with the malt used in your recipe or the iron content of your water. 

Metallic Off-Flavour from your Malt

Be sure to always use fresh grain in your recipes. Malted barley that has been stored in wet, humid conditions can be subject to hydrolysis - which can result in a harsh, metallic taste in your finished beer. 

Metallic Off-Flavour from your Water

It is possible to get a metallic taste in your beer if the iron content of the water used in your brewing process is too high. Iron is perceptible on your palate at only 0.05 ppm. Check your local municipal water report to identify the issue, and try using reverse osmosis (RO) water or reduce the use of brewing salts in your process to alleviate the metallic off-flavour. Another mineral that can contribute to a metallic taste is calcium, in high concentration, it can have a chalky metallic flavour. 

Metallic Off-Flavour from your Equipment

Stainless steel does not impart metallic off-flavours in beer when properly cleaned. Aggressive scrubbing can lead to micro-abrasions in the surface that can impart metallic flavours but this would require a lot of abrasions. Abrasions on the surface can remove the stable oxide layer from the surface of the stainless steel and if not passivated, can provide an area where rust can form. The oxide layer will reform over time but if the surface is left wet this is a recipe for rust. However, if this forms you can remove the rust with an acidic solution (vinegar, concentrated acid-based sanitiser) and a soft sponge/ brush. Ensure to rinse and dry thoroughly. 

Always store your stainless equipment clean and dry.