Since your ECG and Echo are normal these palpitations are unlikely heart related. Get your CBC and TSH done and consult a physician

Sensing own heart beat is called palpitations. They can be of two reasons , one structural problems in heart which in your can came to be normal Second there can be abnormalities in conduction system in the heart , small pulses of electrical current cause heart to contract and beat. These are generally episodic and can be detected with ECG during the episode. If normal nothing to worry. If having continued discomfort then consult cardiologist who might suggest other tests .