Residents near Warner Bros. Studios Leavesden have been warned about "flame effects" and "gun fire" during late-night filming this month.
The company regularly alerts those living nearby if they are likely to hear particularly loud effects such as explosions or sounds as part of any of its productions.
The first five-night shoot began on October 5, with filming running overnight until October 9.
A letter sent to residents says that the shoots will "involve high-level film lighting, controlled SFX, atmospheric smoke, flame effects, wind machines, bangs, rain machines and general production filming noise".
A further five nights of filming is set to take place next week, running October 12 to 16.
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There will be five other night shoots involving sound effects before the end of the month but the dates for these have not been confirmed.
The purpose of the shoots has not been confirmed, but they could be for series two of HBO's Harry Potter television series, which recently began filming on the edge of Watford.
In January, Watford Borough Council approved the studio's bid to speed up the building of four new stages to reflect increasing demand.
A 600-pupil school was also approved last summer, linked to the multi-year Harry Potter project.
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