2003 Toyota Sequoia

My car makes sort of a choking noise when driving, mainly after changing from the first gear to second, after goes to second gear, noise stays for about 4-5 seconds, then disappears, then comes back after shifting to 3rd, stays another 3-5seconds then stops, and its only when i step on the gas, i recall remembering one technician telling me it was a problem with the exshaust or catalyst converter, is that what it is? thanks