Since ECG, ECHO,EMT normal other possible causes for tachycardia should be evaluated... Other possible causes are Hyperthyroidism, anaemia,smoking, substance abuse, side effects of certain medications etc...I would suggest you to consult nearest physician for clinical examination and laboratory investigations they will do the needful... With above fallow relaxation techniques like yoga, meditation,avoid alcohol, smoking, coffee, tea...have good sleep and develop healthy food habits..good luck.