Unknown chirrup

Observation - Unknown chirrup. Description: Late dusk on very clear evening. Loud repeated chirrup sounds - very similar to cicada coming from boggy area with yellow flag, willow herb and long grasses. Heard previously in April and now again at end of June. Carries at least a quarter of a mile. Hav

Late dusk on very clear evening. Loud repeated chirrup sounds - very similar to cicada coming from boggy area with yellow flag, willow herb and long grasses. Heard previously in April and now again at end of June. Carries at least a quarter of a mile. Have recorded on iPhone but don't know how to upload. Would love to know the source, please help.

Further research - could this have been a grasshopper warbler?