The effect is basically to distinguish a point in time from a span of time.

A siren wails (once); an engine is revving (for some time).

However, in practice, the real situations referred to can easily overlap. The siren might be wailing for longer than the engine is revving.

In such cases, if the choice isn't arbitrary (free variation), it could be motivated by euphony, screen space, familiarity of the collocation, etc. For example, I think "I hear an engine revving" would be much more common than "I hear an engine rev" because typically revving implies a continuous effort.

One more option that occurs to me is plot moment. If someone reacts to the siren — it has some immediate effect on the action — the simple present seems more appropriate, whereas if the noise is already established or is in the background, the continuous present seems more appropriate.

The effect is basically to distinguish a point in time from a span of time.

A siren wails (once); an engine is revving (for some time).

However, in practice, the real situations referred to can easily overlap. The siren might be wailing for longer than the engine is revving.

In such cases, if the choice isn't arbitrary (free variation), it could be motivated by euphony, screen space, familiarity of the collocation, etc. For example, I think "I hear an engine revving" would be much more common than "I hear an engine rev" because typically revving implies a continuous effort.

One more option that occurs to me is plot moment. If someone reacts to the siren — it has some immediate effect on the action — the simple present seems more appropriate, whereas if the noise is already established or is in the background, the continuous present seems more appropriate.