Postgresql I/O utilization

Question

Newrelic shows I/O pikes of up to 100% reads.

The rest of the time it's close to 0.

It doesn't coincide in time with our cron jobs (database backup, analyze, etc.), also almost no match with autovacuum (pg_stat_all_tables.last_autovacuum).

Querying pg_stat_activity also shows nothing but our simple queries sent by Ruby application

select * 
from pg_stat_activity 
where query_start between '2017-09-25 09:00:00' and '2017-09-25 11:00:00';

And these queries are not even among our slow queries in log file.

Please advise how to identify what processes cause such I/O load.