Warnings apear on page after signing up for subscription

Hi,

I am having an issue with warnings coming up after I have signed up as a subscriber and about to pay via PayPal. The following shows up on the page http://dronespeak.com/subscribe-to-dronespeak/:

Warning: Cannot modify header information - headers already sent by (output started at /home/dronespe/public_html/wp-content/themes/ifeaturepro5/header.php:18) in /home/dronespe/public_html/wp-includes/pluggable.php on line 680

Warning: Cannot modify header information - headers already sent by (output started at /home/dronespe/public_html/wp-content/themes/ifeaturepro5/header.php:18) in /home/dronespe/public_html/wp-includes/pluggable.php on line 681

Warning: Cannot modify header information - headers already sent by (output started at /home/dronespe/public_html/wp-content/themes/ifeaturepro5/header.php:18) in /home/dronespe/public_html/wp-includes/pluggable.php on line 682

I have attached a screenshot as well. Is this a membership plugin issue or does this look like a issue with the WordPress Theme I may be using? Any help would be great. Thanks!