I have a 1999 ES 300 with 99,000 original miles. I am the original owner and have never had any problems with my Lexus. However recently, I have noticed that when accelerating from Stop to 40 miles per hour I hear a low "roaring" sound like the engine is working really hard. During this period I have limited power. It finally kicks in however my car never did this before. It has been going on for a year now. The car still runs great and just passed smog so I can't believe it could be a catalytic converter. What are your thoughts? At first I thought it was exhaust but now I am a little concerned. By the way I have not changed the timing belt yet...Would that make a difference?
It's a little tough to figure out going by your description.
Power loss up to a certain speed can be a misfire that stops at a lesser load (higher speeds). Does the engine also feel a little rough up to 40mph?
An exhaust restriction would typically slow you down through all speed ranges, but sometimes power can seem almost normal when you hit the next gear. Same thing with a lack of fuel pressure.
The timing belt skipping a few teeth would be more prevalent -- rough idle, backfire, etc, in addition to the lack of power.
And, a transmission problem is still on the table.
Maybe give us some more info to go by.