If this vehicle has a manual transmission; the clutch may be slipping. If you have a tachometer; this would be confirmed if the tach needle moves higher while the speedometer does not go faster. If this vehicle has an automatic transmission; it may be low on fluid. Please note that the automatic transmission fluid level in many cars can only be checked with the engine running; by following the instructions on the dipstick. An automatic transmission which is failing may make such noises even if it is full of fluid. Have the transmission inspected by a competent transmission shop which does not advertise on radio or TV.
If your PT cruiser has a supercharger; it may be going bad; and should be checked by a shop which is competent in diagnosing supercharger problems.
If you do not know of a good shop to diagnose or repair these problems; please click the "answer this question" buton below this message, and type your postal zip code in the box which appears. Then click the "submit answer" button. I'll try to recommend some good local shops.