I experienced the same thing as @sami_ootb. It is starting to appear after upgrading wordpress to version 4.0
I'm using the W3 Total Cache plugin.
One site explain: WordPress Heartbeat and heavy admin-ajax.php usage
The WordPress Heartbeat API uses /wp-admin/admin-ajax.php to run AJAX calls from the web-browser. Which in theory sounds awesome, as WordPress can keep track of what's going on in the dashboard.
However this can also start sending excessive requests to admin-ajax.php which can lead to high CPU usage. Anytime a web-browser is left open on a page using the Heartbeat API, this could potentially be an issue.
Could Heartbeat that caused it?