Chest pain, palps, cough, rapid heart beat. All negative MRI/A, CT, ECHO, ECG, XRAY, BLOODS. BUT Symptoms improve with Diltiazem Tildiem. What is it?

Arrhythmia. Arrhythmia or an abnormal heart rhythm will improve with Diltiazem. You may need a Holter monitor trying to understand if that' s the case. .
Likely benign. I'd likely use up to 25 mg 3X/day of Lopressor (metoprolol tartrate) on an as necessary basis t control symptoms. 1st do 14 day event recorder to identify the arrhythmia (irregular heartbeat). See website. HRS, MD, FACCO.