Mass lesions intimately related to the carotid bifurcation include lymph nodes, glomus tumors and schwannomas. Given that this is an isolated lesion, LN seems unlikely but possible. Glomus tumors are vascular and contain flow voids (salt and pepper appearance). Schwannomas tend be a well defined mass as in this case. It is likely arising from the vagus nerve given its location posterior to the carotid bifurcation (the vagus nerve runs along the posterior aspect of the carotoid sheath).