In this 1955 teshuva written to Rabbi Ephraim Greenblatt of Memphis, Rav Moshe addresses the minhag brought down by the Levush of saying ושם רשעים ירקב when hearing the name of Haman being read in the Megilla (obviously, a custom related to our custom of making noise). Rav Moshe approaches the question from the perspective of whether or not this is considered to be a הפסק, and uses the sugya of whether or not one can be דורש during Megilla reading as a template. However, he ultimately concludes that it is not a sufficient model.
In the closing lines, he suggests that the minhag of the Levush is based on the view in the Yerushalmi that Rav would say ארור המן and since that would not be considered a הפסק, since it was on topic, then neither would ושם רשעים ירקב. However, the Rosh writes that Rav would only say it after the Megilla was read, and thus since there is room for doubt, Rav Moshe advises against doing this.