Toyota Repair: engine rattling noise on 98 RAV4, toyota dealer, exhaust pipe

Question
There is a rattling noise coming form the engine compartment of my 98 RAV4.  The noise gets louder on acceleration and when the engine is under load.  It commonly disapears when up to speed and cruising.  I thought it was lifter noise.  The local Toyota dealer doesn't agree, thinking instead that it is the exhaust.  He thinks the valves that control the release of exhaust to speed engine warm up are not opening completely.  As a consequence they rattle.  I have just purchased the car and did not notice it on my test drive.  Any thoughts?

Answer
Yes it sounds like the exhaust system may be the cause of the rattle noise, it may be the front exhaust pipe or the catalityc converter which may have a problem.
The last statement which starts with "He thinks the valves"........ until the end of you post, make absolutely no sense to me. Sorry, try to be more specific or have the dealer contact me with an explanation and I will help.